“Every person that comes into your life is there for a reason.
There are no accidents. Our relationships and interactions with other people serve a purpose. That purpose is to help each other grow, become more and help us reach our full potential. However not everyone is going to stay in your life forever.
People who are meant to be in your life will make the effort to stick around. People who are just passing through often will be gone just as quickly as they came. Therefore, do not become attached to people, but instead simply focus on being grateful for your shared experiences, no matter how long or how short they may be.
Sometimes it might be many months or even years before you are able to look back and understand the gift that they were, the wisdom they gave you, the purpose and reason why they came into your life and why they did or did not stick around.