In my opinion, Cowboy Bebop is one of those series that has it all: happiness, tragedy, passion, anger... you know. I agree that it wouldn't have been the same if the ending had been different. I mean, if Spike didn't die, then what's the point? What happens after you figured out you've lost virtually everything?

Some people just can't keep living. There was nothing left for him, aside Faye. But even that would have been a rocky relationship after Julia's death.

cowboy bebop is not a sad series...its a GREAT SERIES...although other characters kinda have to die

Is it sad because Julia die?

i think Spike feels the more sadness than we are, julia dies, then in the movie Spike said he fears of loving again, he's unlucky in terms of having a girlfriend.