Steve:
His for You- Sweetheart
Why- It simply came out of his mouth for the longest, even when you were just friends. It bothered you for awhile, but you finally got used to it.
Yours for Him- Darling
Why- You found him to be the kindest, most darling man you'd ever been with, and you couldn't help but call him your favorite nickname.

Tony:
His for You- Babe
Why- Tony, being Tony, had to give you a nickname that he thought would make you feel... Ahem, aroused, when he spoke it. It never really worked on you.
Yours for Him- Hon
Why- It started off as Honey, but you didn't feel like using all those syllables, so you shortened it, and it still had all the love that Honey did.

Bruce:
His for You- Gilligan
Why- You're a klutz, and he knows it. Plain and simple, you break the unbreakable. You also lose things and take sarcastic comments seriously. (Excluding your own) And you really love Gilligan's Island.
Yours for Him- Softie
Why- When you call him this, it defies  everything he thinks about himself. It also confirms what you think about him, and you like that he'll believe you when you say it.

Clint:
His for You- Peeta
Why- You're pretty strong, like Peeta, and he thinks that Katniss totally belongs with Peeta. You actually like Gale with Katniss better, but you go along with it because Peeta's cool, too.
Yours for Him- Katniss
Why- Basically the same reason as his, except you think he's an even better archer than Katniss.

Thor:
His for You- Fair Maiden or Milady
Why- He's Thor. He doesn't even really consider those nicknames, he considers those to be truth.
Yours for Him- Either his name, or MC Hammer
Why- You aren't that big on nicknames, so you only call him MC Hammer when you're wasted.

Loki:
His for You- My Queen
Why- You are his queen, in every way. He simply wants to pamper you and love you with everything he can.
Yours for Him- Grumpy
Why- Sometimes, he can get a little demanding and when he doesn't get it, he gets very grumpy. So, you have nicknamed him after the dwarf.

Bucky:
His for You- Firecracker
Why- You are an extremely outgoing person, and have some slight anger management problems... Emphasis on the anger.
Yours for Him- Bucky-Bear
Why- To be honest, neither of you know. You got drunk, called him that once, and he's been your Bucky-Bear since.

(WOAH GUYS TWO IN ONE DAY HOW AWESOME IS THAT?)