We first see McClane lose his cool when talking to Holly before the party. What starts off as a romantic rekindling turns quickly into a shouting match, because he can't keep a cool head. What separates McClane from every other "loose cannon" cop, is that he acknowledges it as a problem. As soon as Holly leaves, he chastises himself and starts banging his head against the wall, carefully retaining audience sympathy after a jerk move. If he was an unrepentant pig, we wouldn't want him and Holly to end up together. If he was a saint, there would be no character growth. Instead, we have a man who, when things look bleakest, says "She's heard me say 'I love you' a thousand times, but she's never heard me say I'm sorry.'"