But not necessarily. Marijuana is a chemical substance that interacts with our brain in such a way that it can alter our perception and cause physical and psychological side effects. Some perceive these side effects to be positive and so they continue to use Marijuana, but some times the side effects can be negative and include problems like depression.
When some one takes marijuana they feel an euphoria never felt before, happiness no care in the world probably, but when that affect finishes its over, the depression comes like crazy. The pain they were trying to escape comes back worse than before. and when it come back you probably feel worse and that feeling will make you depressed.
Marijuana is something that a lot of people use to block out things that they do not want to deal with. When they come out of that high, they don't want to face the world and their problems. This makes them unhappy so they continue to use the drug as a sort of crutch.
Marijuana relieves depression, at least temporarily. If you don't deal with your depression, it will only get worse. But Marijuana isn't the cause of your depression. You were already depressed. It is only after the Marijuana is gone and you have to face the world sober again that the depression comes flooding back. It only feels worse because you felt so good before.