One common misunderstanding that many people have about depression is that they think once you're happy, you'll forever be happy. That's not how it works and really honestly not how it will ever work.
Depression is something that comes and goes as it pleases. Something slight can trigger it such as, schoolwork, problems with a girl/boy, insecurity, anything really. Depression really doesn't care who you are or what you've been through, it will haunt you sometimes and I wish people understood that.
You have your happy days and you have your sad days. It truly is a struggle and I've watched people go through it and tear themselves up about it. People get so angry because they don't understand that depression isn't just "feeling sorry for yourself" it's exactly what they feel. Telling them that they're just feeling sorry for themselves is just what brings them down even more and makes them feel worse about themselves.
I cannot say it enough, depression isn't something that just goes away. Depression will come whenever it wants to because of the slightest thing. Don't punish people for that. At least try to understand that, because it's just as hard for them as it is for you.