You never know what someone else is going through.
You never know how much a kind word or thoughtless remark will affect someone.
You never know what you mean to others, and how much you can hurt them by your actions.
I recently trained about 11 officers at a 15-person police department (last Thursday), and heard on the radio, by chance, that one of them died by his own hand this weekend, after taking the life of his wife. That prompted some soul-searching on my part. I am not naive enough to think that I should have known what was going through his head, or would have been able to prevent it in any way, but made me think about the impermanence of life itself, and how those whom he has hurt by his actions will be affected forever. It will never be forgotten, by his children, or theirs, into the future.
I explained it to my son today. I explained that things like this are why I try to always tell them I love them, that I always think that, if this was our last moment together, that I want them to remember me well. And how much I hope that he learns that the little things in life matter. They do.