When you look back on all the promises you made to each other, that you would love each other forever, grow old together, and so forth, it can make your whole situation seem all the more confusing and unimaginable.  You might find letters, objects, and other things that swear eternal fidelity, and see these as proof that you are not crazy, that you should not have been dumped.  The only alternative in your eyes is that the whole thing was a big lie.

The Clash knew this feeling too-- "Train in Vain" is an anthem for the dumped, and it can feel good to sing along as Joe Strummer bitterly laments, "You must explain my this must be / Did you lie when you spoke to me? / Did you stand by me? / No not at all."