The comic books had time, though. In the movies only Steve and Bucky and then Tony and Rhodey had that time. Friendships take time. Steve and Natasha had some side missions together and the time to get closer as friends. They couldn't really give Steve and Tony that time because Downey is a major actor and if he'd been in Cap 2, the movie would have become more about him. And Cap 2 needed to be about the Steve and Bucky friendship.