Why does depression come back after years of being completely happy?

You can't really "cure" it, only cope with it. It will always be a part of your life, even if its not the conscious part. Given the right circumstances, it could suddenly come back even if you've been happy for years. The good thing is that at least you've dealt with it before and can more easily cope the next time around. If it does come back and it really stumps you, get help! There is no shame in seeking therapy!

Maybe you have not truly dealt with the emotions that caused the depression in the first place. So when there is trigger it is easy for the depression to come back.

Depression is a serious mental illness. There is no fool proof way of making it go away. You can be "happy" or feel content for a while, and then depression can come back. It is not biased.

It usually happens if you try to smash down unwanted feelings and situations without trying to do something about them. You can't hide from your hidden feelings forever, they will always come back even if you don't think about them anymore.

Sometimes we fall back into our old habits but it's okay as long as we learn from it and move forward.

There are still deeply ingrained beliefs and old neurological pathways that fire off emotions that cause you to be depressed due to triggers.

Nobody is the same. There is no one answer fits all. If you are experiencing feelings of depression perhaps you might want to seek help through a counselor or therapist who can help you to sort out your feelings.

Well there can be many reasons for that, but the most reasonable is that depression is shaped like a boomerang

There may be triggers that can form depression again. You have to learn to over come issues effectively without it triggering negative thoughts

Depression cannot be cured, we can learn to manage it but we're never fully recovered per say. It's important to know that there will be times like this where there are episodes of depression, and sometimes episodes where there is not. All you can do is try to manage those episodes to the best of your ability by seeking help if and when needed.

may have several reasons, like, maybe you did not really get over that depression you just tried to trick yourself and it worked only till now. Or the tings that caused depression to you came back again?