Actually, their music wasn't always like that. The further Barrett travelled into insanity, the more depressing Floyd's themes became.
Comparing it to my and the current American generation's music that is/was, with some exceptions, either whiny and self-aborbed, or angrily rebellious, I find myself thinking: Music for Grownups versus Music for Adolescents. Look at the pictures, for the past what, 30, 40 years -- the "serious" young rock band poses for the camera, nary a smile among the musicians. The setting is gritty, somber. "This is serious work that weighs heavily on us," they seem to be saying. Dopes.