Maslow's hierarchy of needs, maybe?
A few people I know have said that they sometimes feel depressed if they don't eat right. Getting regular exercise might help, too, for the same reason.
Depression can also be caused by being unhappy with your situation in life - maybe you don't feel like you have enough freedom, or you get a constant feeling of boredom or monotony regarding your day-to-day routine, or you feel like your life is stuck in place. Sometimes feelings like that are also coupled with feelings of anxiety or fear about whether or not you can be responsible enough and stable enough to deal with the changes in your life that you want to make, which can make things even worse.
I've even heard of
existential depression, which I don't fully understand - it sounds like depression with no apparent cause, as though one is at odds with life itself. But this seems like the kind of thing that someone might easily misdiagnose themselves with.
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