Need help to know
Short answer is that i don't know why you're depressed, because i don't know you beyond the 4 words you wrote.
You may not consciously know at this point why you feel this way, but with help from a therapist
and/or psychiatrist, you can find out and deal with it. Sometimes talk therapy will give relief; other times medicines may be needed too. You can start with your physician & go from there.