I don’t know what it is about love. Seems like that is the only thing artists can sing about.
However, from human behavior perspective it makes sense. Everyone has experienced, or wants to experience love, and or a relationship sometime in their lives. This makes it a shared experience that everyone can relate to. It also evokes strong reactions that other themes can’t. We all have shopped at a grocery store but I don’t think a song about shopping at Safeway would evoke the reactions that Taylor Swift’s Love Story does.