p.s. There's also another way to look at it all. Why ask (and torture yourself) "Why only on Christmas"?

Let's look at it this way. Many working people live far away from their hometown or parents' home. They might not get to see them once a year. So, I guess the "Christmas is a special, precious time for families" comes from that logistical perspective. Then, it follows that people will tend to or be expected to show more tolerance and kindness than usual. I'm just speculating that correlation as a way to understand the why. I am in no way justifying or validating this behavioural tendency. I agree with you. Christmas should not be an "excuse" to demonstrate or feel love for your loved ones and fellow human beings. Christmas or any similar holiday by another name and religion should only highlight and celebrate the love that already exists.