Stress, relationship issues, work issues. Nothing at all. Depression doesn't flare up. You are just going through the motions and cycles

Causes for depression are many and varied

Having too much time to think.

Lack of love.

The term depression is a very vague and sometimes misused term to describe one's emotional state. Having said that I urge you to look and examine your self thoroughly.
I for example, didn't notice for years that on the days I had little to no sleep my thoughts get dark and I mean dark when I say it.
Back to you, my two cents are; if you neglect your needs, your body will communicate that back to you by having signs of depression.

Chemical imbalance in the brain.

Not being loved for who you are

Triggers
Feeling alone
Not being able to be yourself
Not feeling wanted
It is a very long list

Lots of things

I would say its a fair combination of- lack of understanding, love, compassion, acceptance,security and freedom.

Too much good stuff