Why did Andy Warhol paint a soup can?

Why?..........To provoke a response - like yours. Modern art is not often about asthetic beauty but more about evoking emotion, discussion and thought. Looks like it worked!

The majority of Warhol's work was screen prints, with his stuff such as soup cans, brill box sculptures etc, he was making a statement about mass production and consumerism. He was a commercial illustrator, so used to draw and paint stuff for adverts etc, and it influenced his style. And as already mention, he liked the soup!

The more relevant question is 'why not'? Warhol was making a personal statement about the nature of art, by specifically choosing an everyday object found in most homes and making it the subject of a formal work of art. It means whatever you want it to mean, but possibly that we sometimes forget to really look at things around ourselves; the mundane is too often overlooked or ignored. What does it mean to YOU?

Art doesn't always have a meaning. Do not always search for rules, interpretations in art. Art doesn't care about grammatics. It's the feel that counts.

Why not?

I guess it expresses his feelings from his memories.

Lots of good answers already, I think that Warhol was also exploring the notion of mass production through art. Not sure that it was a painting, though, I do know he did prints as well.