It is all about balance as nobody is entirely positive or negative we are all a mixture of both....Yes there are some great negative songs mixed in with the great positive ones....we need them all.
Depends on what you call negative......Without negative songs where would Dylan be? The vast majority of his songs were protest or political observations expressing views about wanting changes. Without them he would just be another folk singer with a voice that sounds like a wasp in a jar.

I also think that writing a good negative song is much harder than a good positive song. Telling a story even an imaginary one is easy compared to expressing an idea, POV or protest.
Is it all about commercialism, writing what sells and telling stories...or is there more to being a songwriter?

Whilst escapist uplifting songs and happy footappers are a must...room must be left for other types of songs that move people and get them thinking.

http://www.soundclick.com/bands/songInfo.cfm?bandID=687727&songID=5238055