McCain doesn’t seem..stable, to me. I don’t say that to be mean or unkind.
The world's most powerful political office is an awful, gut-wrenching and miserably lonely responsibility that must be taken on by someone, and when a candidate doesn't treat it as such a character flaw is evident, implying either great immaturity or unfounded arrogance. There is always hope that immaturity in a leader will diminish with experience- but an arrogant leader will never yield to wisdom, and in the desire to rule will bring to ruin those he is supposed to serve.