If you are on a mission to be the best version of yourself, this teaching is very helpful. Other people are mirrors to us. They reflect ourselves back to us. So this is great when we see a quality in someone we admire. Enjoy that! Know that if you are noticing this quality in someone else, you must have it in you!
But when we spot something in another that we don't like, it can be more challenging to look at. Don't let that stop you! This is where the growth happens.
When you notice a behavior in another that is frustrating you, it is possible that you exhibit this behavior in some way, too. So ask yourself, "Where am I being this way in my life?"
Maybe you notice someone acting selfishly ... Take a moment to reflect ... Are you being selfish in some way? The reason it bothers you in another is that it reminds you of a quality you don't like in yourself.
There is no shame here! This is a beautiful opportunity to work on changing it!! Noticing it is a huge step forward!