I have studied dreams for a long time, for my age. For about 4 years now, I have studied, contemplated on, researched and logged dreams for the sake of figuring out why they happen. I have done this for obvious reasons, of course.
In a nutshell, dreams are (most often) simply a reflection of our most subconscious wants, needs, and desires. Not only needs, but the past can get in on the gambit too, as well as fears, the future (or time in general), drugs, eating habits, withdrawals, sickness, or the loss of a loved one.