The Hulk is sad. I deduce this from the fact that he has taken to hulk out in somber grays instead of his more usual jolly green hue, and the fact that in human form he dresses like a homeless trucker and looks like a kicked puppy. (These are probably just Ultimate things, but whatever.)
The Hulk is sad because the Super Soldier serum made "dumb as a sack of hammers" Steve Rogers into some kind of tactical genius, whereas it reduces poor Bruce to something "with the IQ of a shoe". (Apparently, in Ultimateverse, they aren't even going to try
to make that gamma rays thing plausible.) He is also jealous of some thing that Tony has where apparently through the use of nanotechnology he has managed to distribute his neural mass throughout his body—a proposition I have yet to decide whether I find erotic or just squicky—but he figures that Tony can put all that brain to good use and come up with some technobabble to help him with his Hulk problem.
Which, of course, he does, but then promptly undoes irreversibly because of reasons, partly involving the first super villain I am aware of whose backstory involves bureaucratic meddling by the European Parliament (thank you, Brussels!). The Hulk sulks off afterwards and that is the end of that.