I think the better question would be to ask "Why do we suddenly cease to enjoy or partake in things we once were obsessed with?"
It's easy to explain that we do things we like because they make us happy which is basically when your brain releases dopamine that quite simply makes you feel good (it's a natural drug), and that's why you keep going back to that song. That's the closest to a simple science explanation I can give you.
We do things we like because we like it.
But why do we suddenly stop liking that song and start to think "OMG I'm so tired of this song!!" and skip it every time it comes on when it once made us so damn happy?