If you didn't turn the movie off and hurl your TV through a window out of boredom, later on, this makes sense. After Bruce reunites with his ex-girlfriend Betty Ross, the two eventually end up in bed together -- again, because this whiny dude meets women like a character in an Axe Body Spray commercial. It's almost the most explicit love scene in an MCU movie.
That is, until Bruce's heart rate monitor starts beeping, causing him to pull back and remark that he "can't get too excited." Yup, Bruce can't get it on because he's likely to turn into a giant green rage monster and crush Liv Tyler to death. For some reason, Marvel thought they needed to include an entire scene explaining why the Hulk has to remain celibate, lest the kids in the audience lose sleep over it. Though at least this might explain why Bruce is "always angry."