Why do I only think about the good qualities my ex had instead of the bad ones as well?

I personally think we all have to think about the good things we lived with someone, not the bad ones. Even though the relationship didn´t work out, this person made part of your life, and somehow contributed to it. Take the good memories to be remembered, get the bad memories and use it as a learning to yourself, about what you don´t like or what you don´t want to repeat in your life again. Always see life with love and optimist!