Letting go of the past is tough, especially when it involves an event or person who affected you on a deeply emotional level.
And when your relationship with that person comes to an end and you're feeling rejected, it can hurt a lot.
There's a reason why the most popular songs are all about heartbreak. It's because we all know the feeling. Heck, even if you never went through a particularly painful breakup, you can still imagine what it would be like, because the longing for a significant other is a feeling that's so deeply ingrained in pretty much everyone.