this topic is probably the one where most mistakes are made

Okay, first off may I just point out how some people don't do proper research when they delve into season three where we witness a humanity-less Stefan.

Stefan turns his humanity back on at the end of the ninth episode, Homecoming. Do you know why?

Because Katherine talked him into it. Here's the actual conversation:

Stefan: "You wanted Klaus dead for five hundred years. Why would you risk all that, just to save Damon's life?"

Katherine: "I wasn't just trying to save Damon's life, Stefan. I was trying to save yours. Your humanity. Let's just say I liked the 'old you' better."

Stefan: "Nah. Come on, Katherine. You don't care about anybody but yourself. You never have."

Katherine: "You and I both know that that is not true. I loved you, I've loved Damon, too. Humanity is a vampire's greatest weakness...No matter how easy it is to turn it off- it just keeps trying to fight it's way back in. Sometimes I let it."

Stefan: "I can't let it all back in. I don't want to. Not after everything I've done."

Katherine: "Of course you don't want to. But if you don't let yourself feel, you won't be able to do what I need you to do next."

Stefan: "What's that?"

Katherine: "Get mad."

After this, Stefan is still an asshole. Why?

Because after committing horrible acts without your humanity, turning it back on will force the vampire to receive all those emotions all at once.

Remember when Elena turned it back on? Stefan instructed her to focus on just one emotion, the strongest one she had and to cling onto it like a life raft in a storm. Her strongest emotion was anger, as was Stefan's when he turned it back on in season three.

So pretty much after turning it back on, he's focusing his rage on killing Klaus and before that could be accomplished, he settled for making him suffer.
Later on, after being cruel to Elena and finding out she kissed his brother, well we can see those little sparks of self-hatred and pain that he's feeling.

So yeah, he turned it back on but it wasn't because of love, it wasn't because of Elena or Damon or Katherine. It was for revenge. Because Klaus took everything from him, his sobriety, his relationships, his force of will. But to get revenge, he needed to feel, he needed to want it. A vampire's emotions are heightened. Always. And his hatred for Klaus grew bigger by the second, and more powerful than his love for his girlfriend or his brother. Down the line, obviously he starts letting more emotions on and starts feeling guilt and accepts help in defeating Klaus.

This plays into my perception of Stefan.
A lot of people are quick to assume the roles of the Salvatore brothers, Stefan's the good brother while Damon's the bad brother.
This isn't true, probably because neither of them are good or bad and these are facts that you have to accept.

In season 1, Stefan was kind and understanding, he did human things but it was for Elena. He wanted to feel like he had a future with her one day, that was the whole point of their relationship.

But then ya know, season 2, Klaus comes in, steals Stefan away,

in season 3 Ripper-fies him and then makes him switch his humanity off.

The reason Stefan doesn't go back to Elena begging for forgiveness after he turns it on is because his anger for Klaus is much stronger.

You need to remember that a vampire's emotions are heightened, not just at the start but always.

They love more intensely, they deal with grief a hundred times over and they hold grudges longer than dragons. So the hatred he felt just burned a lot more than his love for Elena.

Later on, season 5 is a big crap on Stefan, Silas comes in, he goes through PTSD, and honestly, the boy suffers a lot.

Then season 6 is on and he's moved away and made this small life for himself you know, his brother died, there's no way of bringing him back so there was nothing left for him.
When he comes back is to leave his newly turned vampire girlfriend Ivy with Caroline to train and they find out he was never planning on staying. And it sort of reminds us that he's a vampire, and the time he spent in Mystic Falls was like a week compared to the humans' perception of time. [I'll get to that in a minute].

You have to remember that Stefan is just as complex as Damon, he isn't nice out of being nice, he does things with a purpose and sometimes he will stoop to low methods to achieve that.

[Also, if you're looking for another example for heightened  emotions like grief and anger, then refer to Hayley during the first episodes of season 2 of TO.]

She's just been turned into a hybrid and her baby had to be taken from her to be kept safely hidden away, look at how she's processing those grieving emotions and they're heightened so imagine that grief times a hundred. Also, the Mikaelsons have had 1,000 years of practice, but that doesn't mean they can't get dramatic when they want to be. So whenever Klaus feels like he's been betrayed, his heightened emotions make it a bigger deal for him on the inside than most.

TIME PERCEPTION

Now, the whole time perception for a vampire plays into their psyche.

Their minds heal themselves during the tough centuries they experience so they won't go mad with the time passing. This is canon but the only evidence we get for that is during 3x03 of TVD when Stefan tells Elena that 30 years is half her life while it's nothing to a vampire.
And we also have Klaus during 4x06 when Elena is under the Hunters Curse and he tells her how he suffered it for 52 years and that "It was the only period of my life when I actually felt time." so yup that was pretty intense.

Now, onto the more complicated part,

Whatttttttt, it's not complicated yet?

HUMANITY SWITCH

The thing about a vampire's humanity switch is that it's mainly used by vampires younger than two hundred years old (im evening it out). Original Vampires and older vampires like Katherine, Nadia, Rose and many more actually keep it on. [refer back to what Katherine said]

When you go back to season one and you meet Isobel and she's like this big bad vampire that's threatening Elena's loved ones, and Damon explains the no-humanity thing to Alaric, I came to one conclusive argument:

Humanity switches are basically training wheels for baby vampires.

Think about it, most older vampires have had these centuries of experiences of being monstrous and murderous, they're numb to it now. It's a tool to help them transition much faster, also most likely so that they don't avoid feeding, so one could say that the humanity switch is a survival tool implanted for all baby vampires. They're dehumanized and it actually starts out quick.

For example, when Caroline was first turned she was a mess after killing someone for the first time.

(though props to my girl for letting that nurse live, she was the first person Caroline drank from and she lived, how many vamps can do that? hm?)

then not just two weeks later she's killing three (or was it four?) deputies in the most stealthy way when they and her mother captured the Salvatores.

So we can gather that vampires get used to their monstrous side a lot quicker and dehumanized to the violence they can cause.
However, everyone crashes eventually. When the emotions come into play it's whether or not the vampire can handle everything they did, the guilt and grief of it all.

It takes possibly three centuries or more until the vampire starts to lose the ability of the humanity switch because they simply stop feeling guilty for their actions. It's confirmed that after 500 years it's basically impossible to turn it off, or at least extremely difficult.

Vampires like Isobel (a baby vamp) and even the Salvatores, this also includes Elena and Caroline, obviously the baby vamps of the bunch, they've all turned it off, and except for Enzo, the only vampires known for turning it off. Since no hybrids have turned it off. Elena couldn't deal with her emotions and neither could Caroline when it came to her mother.

Now, I'll get into the deep backstories of their use of the humanity switch.

- Damon used it to leave his friend behind to die in a fire so he could escape, he used it as self-preservation. And while he becomes a bit more maniacal, there isn't much different about Damon really, he just cares less. Damon, while being selfish, self-reliant and honestly a prick, actually manages his vampire nature just as well as Caroline does. All in all, he's a pretty good vampire, his control is in check and he actually likes being a dead un-dead living person eternal stud thing.

- Stefan actively uses the humanity switch as such a cop-out. Like when he can't deal with all the horrible things he's done, he flips it and doesn't feel a thing. He doesn't feel guilty about flipping it so he doesn't care for turning it on. It's basically him just pausing his emotions before giving in and feeling immense guilt and even more guilt for all the crap he did with it off. It's literally just him saving himself time, and it just keeps going in a circle. Stefan is as bad a vampire as Elena was when she started out. He's just had a hundred plus years to deal with it.
If Stefan had two guys fighting over him and special blood that a thousand-year-old maniac was obsessed with and wanted to keep alive then he would've gotten the cure a lot earlier.

- Elena, well, we all know how that turned out, but whoever says that she just becomes a carbon copy of Katherine is very mistaken, if you'll just look below:

That about differentiates them perfectly.

welp, just remember these facts before writing the humanity switch wrong okay folks?