The biggest cause of depression are the worries, and not having the ability to be able to visualize its possible solutions, that's when the person begins to enter that state of depression that turns out to be worse, since that way is less likely to arise any ideas.
It doesn't have a specific cause since it can start with anything. Some people are too sensitive with bad vibes while others can cope very well with any negative emotions they come across with. But depression is very real and people undergoing such state should be given attention since it often leads to the person's demise if left undealt with.