I'm not sure if it's spielberg's true philosophy or if he's just catering to what he feels the general movie audience wants, but his two recent movies Minority Report and A.I. make me wonder what he was like as a kid.
What makes the most logical sense to me is that he must have seen a lot of horror and sci-fi tragedies in his childhood, and he decided that they were just too darn depressing and he's been psychologically scarred for life. So his therapy is to make these movies with enormous potential - great stories, suspense, and imagery - and then ruin them in their last half hour with a silly happy ending.
I mean, thank a random deity that he wasn't directing Farscape 3.5 (...Different Destinations). One of the reasons that ep is so brilliant is its tragic outcome.
Oh well.