Depression is a mood disorder. It's a feeling of sadness , may be due to bad relationship , due to professional downfall, disputes in family , Unsuccess,
These days it's very common and important reason behind it is Social Media...
Sometime it is temporary and people get healed and come out of it and sometimes it become difficult to overcome it and under some extreme circumstances people take action.
According to astrology..
Moon represents your emotion.
Mercury shows your logical side.
Sun shows your will power and confidence.
So, in your horoscope if these planets are badly placed , you may come across low feeling.
Any malefic planets connected with moon also give mood swings ..Poor positioned moon can cause emotional problems.
Definitely your planetary position may guide you too.