Why do I get depressed on happy events like my birthday and weddings?

Weddings are happy occasions, but the physical and emotional demands of a wedding can provoke your body’s stress reaction, releasing damaging stress hormones into your brain and triggering your depression. Some other pleasant occasions like birthdays, anniversaries, and vacations can initiate the stress response as well. It really does not matter whether an event is pleasant or unpleasant; if it triggers your body’s stress response, the resulting brain injury can activate your unipolar major depression.