No one knows for certain. It is the responsibility of the Malaysian Government, under Annex 13 of the international aviation treaty arrangements to investigate the cause of MH370's disappearance.
Was the MH370 pilot depressed?
Friends of Zaharie Ahmad Shah, who was the captain of MH370, told The Atlantic that the 53-year-old pilot was depressed and lonely, engaged in one-sided flirting with young women on Facebook, and spent much of his non-flying time pacing empty rooms inside his home.