Maybe because he realises that the best films he was ever in were the Bill and Ted ones and that time seems to have shown that it was youthful enthusiasm more than any comedic brilliance that made him carry his role off. When even the best actors are capable of being witty and lighthearted in interviews, the fact that someone who seems to lack any spark in interviews is being paid handsomely to entertain people is incredibly grating, particularly as much better actors who've been in much better films than he has go out of their way to try to be light hearted.
His girlfriend died... he was having a child to that died. I think thats enough to be depressed for a long time in my opinion. And on the Christian Bale side I really dont like him as an actor i really think his performances are stale and i cant stand his voice, he especially sucked in Terminator Salvation. But apparently he is very good in American Psycho so I need to check it out.