Because you choose not to let go. Now that you know what hurt feels like you definitely know what feeling good is all about. So make up your mind. Personally I choose to feel good myself. You must work on it.
Not understanding all of it, not getting professional help, will keep it painful ..resolution comes in time & it takes work.
It sure would help if u could think beyond your self!!!At the core of it, depression is very very selfish!