Similar movies?

American Graffiti
Happiness

If you want to look outside of Hollywood,

Ajami (Israel) is a good one.
City of God (Brazil)
Gomorrah (Italy)
Amore Perros (Mexico) although I wasn't a big fan of this.

Cloud Atlas (2012).

Another Altman film, Nashville, might be considered similar too. And it's pretty good.

I think most of the obvious picks have been posted but here's a few others....

Me and You and Everyone We Know

The Lobster (this movie is a comedy, a really odd one though)

American Beauty

The Kids Are All Right (2010)

Probably not the most comparable but I might as well throw these out there... Magnolia is a rare piece of film. It's hard to find anything like it. The Lobster is kind of like this too, very rare, not anything like it that I know of.

You already named a couple I was thinking of, but I'd also suggest Iñárritu's "Babel".

Very late to this question, but Grand Canyon is definitely similar.