They Give Up Without A Fight
This is undoubtedly one of the most obvious reasons that a person is mentally weak – a person that is strong on all levels would never give up too easily. Only weak people lack perseverance, and they forget that success will never come to those who give up so easily. Regardless of how difficult it may be at times, one must always fight for his goals: if you give up and throw in the towel before you even start the battle, then the chances are that you are mentally weak, and this may take its toll on your life at a certain point.
This, of course, does not mean a physical fight, but any challenge in life that demands your resistance. Life is full of problems. Majority of the people have a rough pass. Giving up without trying hard means that you are mentally too weak to overcome barriers.