I feel like nothing really makes me happy. I feel content at times. But I would not say that I am sad. It's like the past several years I'm just existing. But why even when things are good?

You can feel down for any reason at all, down to mundane things. It is a symptom of depression. Our emotions can hit us at the best and worst of times and can completely change how we our feeling that day. It is a chemical in the brain that makes you feel this way, or lack of chemical in the brain. Have you ever sought out professional help in the form of a doctor, or spoke to any close peers about what you are feeling? It is hard not to be stuck in this situation but be proactive and try to move forward even though it may seem tough.