Most Helpful Guy
they are afraid to be alone usually OR have low self-esteem. Either way, they put rose colored glasses on and pretend that the other person needs/wants them. So they just ignore the obvious and HOPE the other person will eventually change if they just LOVE them enough.
Most Helpful Girl
They have no respect for themselves or don't value who they are as a person. Maybe they are just blinded by some quality about that person (money, looks, success) and they just put up with it because the alternative isn't something they could handle/live with.