The problem is it's very hard to tell when a superhero is recklessly endangering himself versus when he's accepting the very real dangers of the job. Tony, who is the embodiment of reckless on a good day, doesn't know if Captain America is throwing himself into his work because it's his work, or because secretly he wishes he were dead.

It scares him. And the fact that it scares him also scares him. Tony is used to being a shallow jerk; he's very good at it. Caring about Steve to the point that he worries if the guy goes off his own personal radar for more than an hour is seriously fucking with him.