It's the words and they needn't be sublime like mines, but they do need to reach the parts that we most identify with in everyday life. The pulse of the song is important too. It should beat like a song in silence and have great lyrics. There should be something just touching that melt the heart like 'stay' from Rihanna or appealling to the masses like 'No woman, no cry from Marley.
I'm trying to remember the name of the guy who wrote an incredible love song which is always in the top 10 most popular love songs.. Ah! John Legend, is it?Check out the Beatles or some of Lionel Ritchie's.