Tony hates dumb people.
That alone is a major reason he lacks in the friend department.
Not girlfriends.
Not friends with benefits.(He has plenty of those)
Not the usual ass-kissing, suck-ups that tolerate him for money(He has plenty of those too.)
But real, honest friends that won't put up with his crap.
Like Rhodey and Pepper.
It's not just his narcissistic personality, his overpowering genius, inability to handle serious situations, or need for reckless endangerment that keeps them at bay.
It's the fact that Tony hates dumb people.
Considering he is THE smartest man alive that narrows the potential friend field by a considerable amount.
In a conversation there is only one pace, his. And its pretty damn fast.
Keep up with his genius mind or get left behind.
That's it.
No room for error.
So that's why he doesn't like Steve at first.
It had nothing to do with the fact that his father practically worshiped the man with a sculpted stature that could challenge Tony's adimatium clad confidence or that he spent more time with the super soldier than his own son.
No resentment there.
Tony just thinks he's dumb.
Tony knows he is.
A man whose mind is still in the 40's couldn't possibly comprehend the complexities of modern-day society. He's not in Tony's League, he's not in anyone's, he is simply lost.
Tony sees no interest in conversing with him or any attempt at friendship.
It would be a waste of time.
He would gain nothing.
So he thinks.
Until he begins to talk to the man known as Captain America.
At first he'd been intentionally avoiding speaking with him, he often spoke to him, tossing blonde jokes, references, and innuendos Rogers could only hope to understand, but as far as conventional conversation on a topic of mutual interest(which they had none), he avoided.
Then they were arguing one day, big surprise there, and Steve said something that made Tony shut his mouth, stop, and think. That in itself is a sizable accomplishment considering Tony doesn't stop to think(or shut his mouth)often. He is genius. The facts come together into logical sentences accompanied by his patented sarcastic spin at light fast speeds. The words immediately follow.
Its natural.
So when he actually stops to think, he thinks that maybe good the Ol' Captain isn't s dumb as he seems.