Marijuana has chemicals (THC, CBD, and many more) that affect your brain receptors, almost immediately. Last time I checked, Depression is caused by chemical imbalances in your brain/body...For habitual users who have been smoking for decades, just wait...Something serious you have never experienced, but feared to happen, actually happens...Your chemicals will react and throw you into depression without even knowing. Marijuana will amplify the chemical imbalance. Good luck trying to quit when you have to go see the doctor, and he/she tells you the root of your depression.
Marijuana is a chemical substance and any chemical substance that can alter brain chemistry can cause adverse physical and psychological side effects. Those side effects may include depression. While Marijuana is valued by many because it is relaxing, all people who use chemical substances should be wary of their potential for adverse side effects.
While I do believe that it's not the actual drug effect of Marijuana itself that would cause depression, I do believe that the bodily effects of having used it as a method of a happy stimulant can cause some people to become depressed, yes. What goes up, must come down, as the saying goes. And Marijuana while may be pleasant on one end, may find the user feeling sad when the effects are over.
Everyone reacts differently to chemical substances, and in some cases, there will be people who use pot and end up feeling depressed. It can be difficult to determine exact causes because sometimes people use drugs when they are already depressed. But it is possible in some cases for people's emotional states to be dropped down because of marijuana use, even if this is the minority of cases.
It may not directly cause depression but once the high is gone it can be depressing. Also many people become addicted to it and spend most of their money purchasing marijuana. This can lead to financial problems which are very depressing. So although the initial effects of marijuana are not depressing, they could have depressing side affects.
I use marijuana on a regular basis and all it does is stimulate your brain and body by making you feel happy and relaxed. Marijuana helps relieve depression, and in my personal opinion it is way better to smoke a joint a day and feel better than stuff your mouth with those lab-made antidepressants.