Transitions generally trigger bouts of depression for me. I think it's hard to say why it is happening in any one case.

I just joined a new company - more money, less work, less stress, nicer people.

Now depression hit me hard and heavy.

Possible factors(?):

1. Will I live up to what I said I could?

2. My job has always been the basis of who I am - I need to have an identity that is not defined by my career or position.

3. I still cannot feel accomplished - self-worth issues.

I am sure there are more, but I have to go now. Hope this offers insight from similar experiences.