You can look at people walking down the street and they have their angry face on, and you think to yourself, "Why are you so angry? What do you have to be upset about?" And you might even want to say to them out loud, "Gosh..smile a little! Life's not that bad :-) " But I wouldn't suggest it. Because you never know what someone else is going through. Bad things happen to good people all the time.
Which leads me to my next point, sometimes people are upset because something bad has happened to them, and it's not even their fault. And they are angry because there was nothing they could do to stop it. I know our parents told us, "life's not fair" when we were kids and didn't get what we wanted. They were right. Bad things happen to good people, and that will never be fair.
But what are you going to do about? Are you going to blame someone else? Or take the high road, and pragmatically expect that sometimes, life throws you curve balls. It's what you do about it that counts.
