The five universal song topics...

Love... unrequited love, never-ending love, brothery love, end of love (heartbreak songs could probably be a separate category but).
God
Country
Drinking
The Road/making music

really... every song in the world is about one or more of these things... and if you lump God and Country into types of love songs (Faith and Patriotism being aspects of love in a sense); there's really only three.