When you are being plagued, never ask what for and why. You will never find that in the Scripture. Instead, it says: "If somebody strikes you on your right cheek, turn the other one to him. It is really difficult to strike one on the right cheek and should be understood as: if somebody starts to denigrate you or provoke you unjustifiably, that will mean hitting you on your right cheek.
Do not grumble but bear this blow patiently, and turn your left cheek, that is, remembering your past faulty deeds. If in this time you are innocent, then you have sinned greatly in the past; with this you will be convinced that you deserve punishment. Self justification is a large sin."
Elder Ambrose of Optina