Maybe someone could explain my recent experience to me because i really don't know.

My aunt bought a condo. So I went out of town for a week to help her move. I remodeled the bathroom, built some shelves, moved all her stuff and so on. We went golfing, went to the casino, went to dinner with friends and family, went to watch my nephew play hockey. Had a great week and enjoyed it very much. Got lots of exercise, ate good, social activity with friends and family, purpose, got paid, felt really good. This has pretty much been true since last April, living this lifestyle.

Then I go home and got slammed with major depression. In a matter of two days I had all the symptoms. No energy, no motivation, have not showered in a week, don't want to get out of bed, didn't get out of bed until 6pm today. Starting to get suicidal thoughts. I had things planned. I have things I need to do. I can barely brush my teeth. I have said this many times and I swear it is true. It is like I can feel a switch going off in my brain. This has happened to me many many times in my life just like I have described. Durations vary. the switch may turn back on next week.

So how do I go from doing everything right, in therapy, all the right life style choices, to bam, brick wall depression??? Am I just on the pity pot? Is it my fault? I am just not trying hard enough? It is not really a disease? Please someone explain to me how this happens.