"One of the first things Robert said when we were meeting about the movie was “There’s no way Tony Stark gets in a room with Bruce Banner and doesn’t poke him”. There’s no way. He wants to see it happen, he wants to see how that guy works. And that is not only true, not only ended up in the movie but ultimately informs their entire arc, which is one of my favourite things in the movie. …he sets him up and lets him gets to the place where he can decide to be the Hulk."
Joss Whedon on the Tony/Bruce relationship (The Avengers Blu Ray Commentary)