My roommate grinds my gears into a powder.
I'm usually pretty tolerant of noise but he takes it to a whole new level. It's the noise that really messes with me because it keeps me from getting stuff done... but he also throws tantrums, tries to blame me or inanimate objects for petty things, thinks his cat is bad for not wanting to be cuddled all the time (I seriously worry for that cat sometimes), refuses to clean up after himself, and wants my attention all the time.
It's alright if a child is acting like a child, but a grown adult the same age as me (26) acting like a little kid who is totally out of control, and doing so all the time, now THAT really, really grinds my gears.