We only saw the Marvel Cinematic Universe version of Hulk in four movies prior to Avengers: Infinity War (The Incredible Hulk, The Avengers, The Avengers: Age of Ultron, and Thor: Ragnarok), and while that’s not as much exposure as a lot of other heroes we have learned a bit about him.
He typically comes out swinging at every opportunity he gets — partially because he feels caged by Bruce Banner — but mostly because he loves showing off his strength.
Let’s not forget that he tried to take on a fully-powered Surtur the last time we saw him.
Of course, that being said, it’s extremely easy to be brave when you believe yourself to be the strongest entity in the universe.