Laughter feels good. (And it should feel good—unless you're laughing at someone getting thrown off a cliff or run over with a lawn mower.)
It makes Uncle Albert feel good, at any rate:
ALBERT: I can't help it. You can see that. I just like laughing, that's all.
The fact that Albert floats symbolizes the way laughter makes you feel: light and easygoing. If laughter made Albert heavier, until he sank into the earth, it wouldn't quite feel right—it would be bizarrely off-kilter.
Yeah, it's a pretty obvious symbol, but—hey, someone had to step up and point it out.