I've felt the exact same way for the past several years and although it's gotten a bit better lately, I still struggle with trying to figure out why I don't have some of the same joy and happiness that the people around me seem to have. I had kind of always kept the fact that I just felt like "neutral" (not sad or happy) or like there was something missing to myself.
Part of the reason I've felt that way was due to depression and other problems that I just kept ignoring and down playing how severely they affected me. I'm currently being treated for those and it's made a difference for me.
Luckily, I have found things that I truly love and am passionate about that are completely unique to me. I'm a shy person and I feel like that has kind of held me back from pursuing what I want to be involved in, so I was never really getting to do things I wanted to do. I've really been making an effort to get out and try and do things that will make me happy, even if it's by myself.