Friendship is always an interesting concept. Some people you're friends with forever, others only for a small amount of time, and some people you just meet and know that you guys are not going to be friends at all. The crazy thing is we don't meet people by accident.
Whether you believe that it's by chance, fate or by the grace of God that you meet someone, everything happens the way it's supposed to happen. Sure, there are going to be people that come into your life and make a mess of things, and you may ask "why me?". The real question you should be asking is, "what was I supposed to learn from this?".
No friendship in your life is by accident. Each and every person you meet, you're supposed to meet at that exact moment, in the exact point in your life, and whether that person is there to stay for a long time or not. At every point in your there are different people that go with you through situations. Those people are those who are there for you, those who want to be there just to know your business, those who want to be there just to tear you down, and those who will hate on you from afar. Our job in these situations are to figure out who those real friends are.
These is always easier said then done, but real friendships take time and work. If you don't put in the work, your friendships can't flourish. The flip side is that we also need to learn when to let go. Everything that God has in store for us isn't meant for everyone to experience. Truthfully, everything we experience is mainly for you. It just so happens that there are people by your side to experience things too, but they don't experience it like you do. Why? Because it's not meant for them.
That's the thing about friendship; your real friends should be the one's who encourage you, support you, and challenge you. Anyone doing anything short of that, doesn't deserve to have even a minuscule piece of your life. They aren't worth your tears when they finally show their true colors, and when they do, believe them. Every stage of your life and everything you experience and everywhere you end up, not everyone you thought would be there is supposed to continue on with you.
So, when someone walks out of your life without any explanation, don't chase them. Don't beg for them to come back. They were removed from your life for a reason and you just have to trust that God has a better plan for you. If they come back later, then you know that before wasn't the right time or God is trying to teach you something so you get it right. Everything that happens is in God's perfect timing, and we should "not be anxious about anything, but in every situation, by prayer and petition, with thanksgiving, present [our] requests to God. And the peace of God, which transcends all understanding, will guard [our] hearts and [our] minds in Christ Jesus." - Philippians 4:6-7