There are a lot of possible reasons. When we start becoming young adults, we have to start building our own worldview. We have to start thinking about what it means to be alive, and that can be scary. Talk to your family and your friends. They may be feeling the same way.
i dont know why your depresed but if you do these things,stop them all and you will feel much better.stay away from people who talk alot or backbite,gambling,smoking, drinking,arguements,do some exercise,go to a country have fresh air.holiday,look after your diet.