This adage surfaced on a Quora thread about simple statements that have changed people's lives for the better, and it's not surprising it rose to the top. It's easy to go through your life believing that there are people out to get you, but the reality is that their motivation rarely has anything to do with you. They just want what they want, and sometimes that may affect you negatively.
Photo from Game of Thrones.
I think this is important. Even deliberate actions taken against you aren't necessarily against you. For example, an employer may fire you for being too good because s/he is threatened you'll take his/her job. That's a stupid action, but the motivation is self-preservation. It really doesn't have much to do with you.
It can be easy to get upset with others who make choices that affect you negatively, and sometimes that's warranted, but ultimately they're just looking out for themselves. If you can remember that, it can be easier to let go of the little things and choose your battles wisely.