Depression comes in all forms and format, as stated by few in their respective replies, may it be due to job, relationship, self confidence, being introvert, seasonal and you name it. More over the years the society had developed in such a fashion, that it without your consent drive you to life that you choose not to live, where you may lack so many things comparison to another and either to meet those unnecessary requirements be it the best car, good job, good gf/wife/husband etc.. People tent to forget to be happy with what they have and value it. Also I think it is also lack of discipline to your body that it seeks, love your body and eat/sleep/exercise in the best way your body wants it.