Tony
What you can't see (the dark).

Hah, what a coincidence! Now you've got your own little night light!

Steve
Being alone.

Your dad is all you've got, without him you wouldn't know what to do but now you have Steve and you couldn't he any happier. You're also scared of clowns and swirly straws.

Bruce
You're honestly scared of pigs, you think that if you get within 10 feet of one it will chase you around trying to eat you with its other little demon pug friends. You're also scared of geese because their long necks are creepy and the way they hiss and chase after you seems most sinister.

Pietro
Shadows in the dark and home invaders. He learned that when he had a little romantic surprise waiting for you. He was sitting in you twisty-turney computer chair in your room and you walked in and he swiveled around and said, "Helloooo gorgeo-"

He didn't get to finish because you screamed and kicked him in the nuts as you screamed DIE DIE DIE.

...yeah he learned the hard way

Bucky
Cannibals. He thinks its funny so when you get home late from work he'll hide underneath or behind something and grab you and start to knaw on your neck or ankle playfully as you scream and then scream more becasue he "ruined" your evening. He always made it better though by giving you a nice long and relaxing back massage.

Thor
The Boogeyman. Its totally childish and cliche but you can't help it. Thor tries his best to understand but never fails to make you feel safe by yelling at the closet every night.

Clint
Birds. You're dating a birdman and it just so happens that you are petrified when a bird crimes with 10 feet of you. Especially pelicans... The only bird you love is Baby.