This question is usually answered by stating that the reason we use substances is to escape from our problems.That rarely felt true, especially when I was younger and enjoying life. If I had a dollar for every time I heard that I would be a rich man. Escaping problems is not the reason human beings from time immemorial have used substances. We use substances in order to alter our spatial-temporal continuum in the world. This change in our spatial and temporal continuum occurs every time we use substances, no matter the reason or occasion. Whether it is too enhance our enjoyment, drown our sorrows, or even occasionally run from our problems, the effect of drug use is to change how our body feels in space and time.
Marijuana is good example of this concept at work. Imagine smoking your first joint. There is one hit where your viscera knows that it's there. There is a clear perceptual shift and you know you are high. The world unfolds before you in a changed manner. The body flows through time with a different cadence. The body is attuned to the world, and the world attuned to the body in ways different then before the use of the drug. Relapse, for instance is apt to occur when all is well. No apparent reason. No particular problem. An image, thought, desire flashes into consciousness and the body becomes uncomfortable, to the point of needing to be satiated, calmed may be a better word. Hunger, anger, fatigue are classic triggers for relapse because each to an extreme shakes our attunement to the world .This, for me, is a crucial concept for understanding addiction or any substance use. Different drugs obviously effect our attunement in different ways which I will discuss shortly.