Sometimes it would be hard to realize that you aren't much connected to some people who were dear to you. Some leave you for a cause and some may build reasons to get away. Whatever it maybe, don't stress yourself for anyone and accept the thing that you're not the same for them. Don't worry, it happens not only with you but to each one out there. Simply, Let them go.
You should rather look around for those who are with you, who wants you, who loves you, who is willing to stay for long. Shift your focus from what's gone to what you have.