"Say you're sorry when you hurt someone" - All I Really Need to Know I Learned in Kindergarten by John Fulghum.
This is a wise book which reminds me that sometimes the most basic things are forgotten. There are quite a few pearls of wisdom contained in its pages: Take a nap every day, share everything, don't hit, say you're sorry when you hurt someone and my personal favorite: warm cookies and milk are good for you. These lessons in etiquette are taught to us at a very young age and are the basics for how to take good care of ourselves and treat others. Somehow along the way, possibly because of all the other information we acquire, we sometimes forget these simple rules.
It's impossible to make it through life without ever saying or doing something that hurts someone you care about; we are all human and make mistakes. Whether we say something without thinking, do something without considering all the consequences or forget to do something as an act of omission, for one reason or another we've all been in that position. It never feels good to realize you've hurt some one's feelings.
Each person sees the world through their own unique perspective and it would be unrealistic to think that from time to time someone else's path, actions or thoughts won't be at odds with our own. The best I personally can hope for is that I never intentionally do something to hurt someone else: Malice.
Naturally, there have been and will continue to be times when I've done something to hurt someone I care about. The only solution to this, as words and actions can never be undone, is to offer a sincere apology. The power of "I'm sorry" is often underestimated.
I've noticed that often if an apology is offered, it's accompanied by a defensive attitude: excuses and rationalization. In my eyes this takes away from the sentiment. I'm not saying to appear before the person as a whipped dog, tail between your legs, begging for forgiveness. That's up to them. A simple "I'm truly sorry that I hurt you" should suffice. We have no control over whether our apology is accepted, and can never expect the other person to understand our position, just as we will probably never understand theirs.
As John Fulghum says: "nobody's perfect". If we can remember this of ourselves as well as others, we'll be a lot more realistic in our expectations. This just might be one of the most important lessons in the book.