Two factors in my view:

1. You don't remember the good dreams, because we humans generally focus more on the bad things. Dreams generally don't get remembered very easily, so you only remember the very impactful ones, which would thus be the bad ones. It is possible though to want to focus more on the good stuff and train yourself to do it.
2. Afaik, you do have more bad dreams if your life is filled with anxiety or danger or whatever. Your dreams in some sense are a reflection of your waking life. So if your life is very stressful, maybe try to change that, and that might also change the dreams.