We only capture the happy moments, taking pictures when we are having a good time. In the other hand, we never do when we are sad or going through a bad day. Hoping we are going to forget these black phsses/moments one day.
Human being tend to forget things, memories; even the good ones sometime, due to the lack of time passing. The beauty about forgetting memories, is like the RAM, in order to have place for new things, you need to get rid of the unnecessary files, same goes when it comes to life.