Everyone suffers from depression from time to time. It's a natural defense mechanism that allows the mind to take a rest by causing an individual to withdraw from reality for a day or two. But for some people, the withdrawal is deeper and lasts longer. It interferes with their lives and can lead them to substance abuse or suicide as a means of escape. When this happens, a person is said to have a mental illness called severe depression.
Mild depression is the most common and can be brought on by both happy and sad events.
A wedding is certainly happy, but also very stressful, and the stress can be depressing. Another common cause is childbirth, which may lead to post-partum blues. While usually mild, it can become severe. Moderate depression, or a feeling of hopelessness, lasts longer and is more intense. Moderate depression is often brought on by a sad event, such as a death of a loved one or loss of a job.