I think depression is probably a bit of a lot of things. I can't speak for you. But, I am a very negative thinker by genetics (siblings all have a tendency to think in a negative way) and upbringing (my parents were rarely positive towards me). I find that my religion (Christianity helps me) but it can also hurt you. I find that I expect too much out of myself in the area of helping others. I mean, if Jesus died for me, I should be willing to sacrifice for others. Due to this, I tend to never feel like I have helped someone and turn into a walking guilt trip. But, knowing that Jesus understands, loves me and is with me always helps me to keep living. As for the neurotransmitter theory, I am not sure what I think. (T has suggested that I take an anti-depressant) I think it is mostly based on how anti-depressants work. They figured that if this is what they do, then there must be a low level of serotonin in the brain.
So, I would say that every individaul's problem is different. On the dealing with emotions, I stick mine in my black hole of denial (Gee, does that sound mentally healthy?) Some people may have a depression that is a stress/life situation problem. For others it is a complex combination of life stress, past experiences making one more susceptible to things and biology. I suspect that you could find someone with a different theory than all of these.
I hope you can find what works for you. As for whether you even have a problem, I suspect that only you can find that answer.