The Right Ones Will Stay

My girlfriend keeps saying this phrase as a declaration that she's not going anywhere. I've had anxiety over people leaving etc. It is a scary thing. They are there one day and gone the next. It's a risky way to do anything if you don't expect them to stick around much. So if you don't expect people to stay, what do you do? How do you cope if/when they leave you? That has been a question I've been wondering myself for a long while. I still don't have an answer for it.