Not every song is but it features as a theme more than a lot of other possibilities because many songwriters write based on their emotions and love is a strong emotion that a large number of people can relate to.
Love songs are commercial?
Listening to music makes me think of people i know so its only logical for songwriters to write about what they know, have experienced, for a lot of people the creative process is way of venting emotions. Doesn't everyone have a few songs that they turn up very loud when they're really angry or upset or completely elated :P
There are plenty of musicians who sing about everyday struggles among other things!
..Political, you see.
Songs about love are popular because that's one of the strongest emotions you can feel, something that most people can relate to, and therefore an interesting thing to write about. Love songs tend to sell well too.
In terms of commercial success, love songs are generally great for capturing the hearts of the public. Even if it's written terribly, it's something that a lot of people can relate to. Similarly, songs about "the club" are popular too, but more because it's something to get people in a good mood.
For some reason, when I try to think of songs I listen to that aren't about love, I end up with "Nan, You're A Window Shopper" by Lily Allen O_o
There's plenty of good songs not about love though, it's just that love is a universal subject. Looking at my Top 50 songs on iTunes that aren't about partying or love, there's topics such as rough urban areas, trying to make a change in life, satire involving the lack of originality in how teen girls act, and for some reason a song about a neighbourhood slag. However, a lot of my songs are about love or partying, and it's weird how these subjects are ones that pop up so often in music.