Everyone that crosses your path does so to either lift you up or show you a weak area of yourself that you need to work on. Either way it is a benefit to you.
I learned this through experience, you may think that some people are there as a friend for life, especially if you've been through a lot together. When in reality they crossed your path to show you that you need to work on something, and for me, I needed to work on letting go. I appreciate and I am grateful for that lesson and the person that was used to show me my weaknesses