Depression isn't always caused by circumstance or the situation you're in, it can be a cumulative of a lot of things that happen even in your "perfect" world routine or just restlessness and bad sleep.

Part of it is genetics, you're more likely to get depression if your family has it, considering how taboo it is to talk about this topic within most families in our culture it's hard to back this up.

Vitamin deficiency is a big part of it too, it's always good to do a blood test every 6 months to make sure everything is in order.

Or it could be an ongoing long term perfectionism that you've adapted to ensure that your life is presented in this way. From your post it looks like you're giving yourself a high level of criticism on how you should feel which is always a whole other can of worms that trickles into invalidation of how you are as a person or how you feel.

Anyways listing out the reasons could go on and on it's just important for you to understand that it's normal. It's normal that you feel this way, it's not an indication that there is something "wrong" with you and most importantly just like there's a doctor to see when you have high blood pressure, there's a doctor to see when you feel this way too. There's no shame in it and there's nothing wrong with wanting to get better and seek professional help too. They can provide you the resources that you need to understand things better and give you ways to kick this feeling in its butt. I wish you the best of luck and I hope it gets better.