Marijuana is one of the most versatile plants on earth. It can be extremely effective to treat mood disorders, including depression, social anxiety, and bipolar disorder. Unfortunately, the United States food and drug administration has determined, without explanation, that there are no viable medical uses for the drug, a finding which has been refuted countless times.
Medical marijuana helps nervous and stressed people relax and mellow out. Yes, marijuana can also impair judgment. But in general, the drug does improve people's moods. It shouldn't replace therapy or nutrition when dealing with psychological problems. However, if nothing else works, medical marijuana is a viable option as THC can do funny things to brain chemistry to help someone relax.
Psychologically marijuana will (if the patient can use it) increase their body's levels of certain chemicals in the brain like dopamine that enhances a person's mood temporarily. However as time goes on especially with regular or over-use of the drug the body will realize that it has an excess of that particular chemical, and in an attempt to control and balance the levels it will stop or slow the production of it. This usually leads to a sort of crash after the drug has worn off causing the levels to drop below where they had started and effectively lowering the person's mood overall until they take more of it. This causes dependence and in the end addiction that can lead to other problems in the future obviously working against the goal.