Why can't I just be happy like everyone else?

The people around you typically aren't as happy as they seem. Most young people do not feel happy, because humans typically outgrow school and childhood before they turn 18. Because of this, older teenagers sometimes feel like they are being restricted. So again, chances are that most people around you feel the same way as you, it is statistically most probable. Try to look forward to something, something you're really excited about. If you don't feel you have anything like that, make something. Think about what you will be doing in the future and how you can do stuff now to help you towards that future. It will help give your life purpose right now. Good luck.