Because he's Steve 'Captain America' Rogers, and when he believes in you, you believe in you too. It's the real super power he's got. That and the mother of all puppy dog eyes in his current tiny form.

"I appreciate it." He says gently. "What I'd like you to do is make some notes, let me know what I need to fix. And then I'll work on it and then you'll check it again. And only when it's alright by you will I shade it all in."