Because a real man will have it naturally in him and 'want' to learn from life/try harder in order to better himself, not be weak by making himself worse and giving in to weaker temptations, selfishness, immaturity and laziness. With the ones who don't try - let them go so they can learn life the hard way. Find someone else who DOES value you. If you give of yourself completely to the 'right' person, they will mirror those values and caring qualities right back to you. That's a keeper.
