I suffered from depression pretty severely during my senior year of high school and the summer before I started college. When I say severely, it probably does not compare to other people here, but it definitely impacted my life negatively on numerous occasions. Hurt friendships, family, my own personal well-being, etc. The biggest reason, I think, why I suffered from depression is my own thoughts. I was the type that had these pre-conceived notions of what I wanted people to say or do, and if they didn't do them, I would take it personally. When something like that happened regarding a close friend, major event, or multiple times in quick succession, I almost surely would be depressed for a few days. In the summer, I was mainly depressed thinking about how quickly high school ended, losing friends, losing my best friend, moving away from home, nervousness about college, all these things.
I'm happy to say that since college, my depression problems have significantly decreased to almost non-existent currently. Main reason, I think, is that I keep my expectations in check. No longer do I expect something out of people that has little chance of happening. Some might see it as a bad happening to never expect anything out of people, but it keeps me happy (though perhaps sometimes emotionally distant?).
To those suffering, stay strong. I'm in no position to really give advice given my situation. However, I do think it's important to sort of self-identify what makes you upset/depressed. Obviously, this does not apply to everyone, or even majority. But for those who have depression spells like I did, try and find the cause. It might be curable by your own volition.