When Bucky realizes, after the near implosion of the Avengers, that Steve is capable of handling his own in a fight, he decides he needs another hobby to preoccupy himself. That hobby just so happens to be pissing off a few Avengers enough for them to fight him. Maybe he likes the thrill that fighting the "Earth's Mightiest" gives him. Maybe he just likes who patches him up after each tussle. Honestly, the answer is obvious to everyone except Bucky dear ol' Doc...