Selfishness is a root cause of much Dukkha. As to being specifically the root cause of depression, I think it has a lot to do with what type of depression it is and I am no expert on various types of depressions. Certainly it can arise when we are too caught up in "me, me, me, my problems, my problems, my problems". When I did the D/D Compassion/Lovingkindness meditation the other afternoon and I was talking about fear, during the 2nd half, I showed how a person is afraid and the reaction to it can be very different. For some it is more of a small thing, for other people it gets more into crying and sitting in chairs, some probably want to escape and some people end up killing themselves or others. There is definitely a lot of 'me', 'my world' there, so if the selfishness you are referring to includes that type of "me, me, I have pain, I don't like the pain, I want out of the pain", if you are referring to that type of selfishness, then yes, probably all types of depression come from that, but we are looking at a different terminology here and how it is used. Certainly selfishness causes much depression.