it's rick's weakness. rick's strengths are shanes weaknesses and shanes strengths are ricks weaknesses. Problem is in this environment shanes weaknesses are actually strengths. the fact he can not care about taking a life to save the ones he loves. Rick still has problems with it and will always be weaker then shane until he can solve it. Ricks getting there though so once he has that down pat then he's the easy shoe in for leader. but so far every single decision shane made in this season has been better than ricks.
sorry but i disagree with you and i believe ricks decisions are more humane and are better. we see how shane has affected andrea, she basically convinced that one girl to try and kill herself. shes a pariah now, she was always kind of an outsider but never as much as after being affected by shanes way of thinking. shane is a dangerous infection within the group that threatens their stability and safety whenever he makes any type of major decision. i believe that shane would benefit more from ricks caution and morality than rick would benefit from shanes lack thereof
i believe that shane would make the perfect leader for those guys rick shot in the bar. hes not an honest and 'good' person. he used to be, but maybe hes lost forever now. rick asked him to "Come back", rick wants shane to be the same man as before all the shit happened, before he changed into the animal that he is now