Many nouns in English can be used in either a count or a non-count sense, and money is no exception. In general, the money refers to specific money, and ∅ money (no article) means money as a general phenomenon or idea.
So when you say "The money isn't the root of selfishness", it sounds like you are referring to a specific sum of money - which you probably aren't; you're probably talking about money in general.
When the song says "It's not all about the money", that's because it's not all about the specific money involved in whatever it is.
This sort of thing comes up a lot - for example, I can say I ate a pie (one specific pie) and now there is pie all over my face (a nonspecific amount of "pie" considered as a general substance). My friend Bill has a philosophy that causes him to not give money to charity (because it's one specific philosophy) but my friend Mary studies philosophy (that is, it's a general concept or field of study). You can even do this with normally countable nouns: The terrorist strapped the bomb to the cow (specific terrorist, bomb, and cow), and after the bomb went off there was cow all over the place (a nonspecific amount of "cow" considered as a general substance).