Oh yes I very much do think it can be related to those things. It is for many people. I have often heard that anger turned inward is depression. it is somewhat simplistic but makes sense to me. I think of anger turned inward as shame. Shame and fear are the two biggest culprits in my view.

I think PTSD, sexual abuse, physical abuse, abandonment, neglect, fear of intimacy, fear of commitment, unhealthy coping and processing of emotions, drug and alcohol abuse to cope, low self esteem, for whatever reason, unhealthy parenting, and so on can and do cause depression and a host of internal conflicts that effect our lives.

Oh yeah and stress for sure and loss.