Years ago when my dad was a young pastor he had a situation where an older man left the church. My dad was upset by this because this older gentleman had been the one to hold the church together while they didn't have a pastor and he had helped to get my dad there in the first place. But, this older man was grumpy when he couldn't manipulate my dad into doing things he wanted him to do, so he left.
My dad called the man who was over him in the mission work he was doing at the time and asked him what to do. He told my dad not to chase that older man down the road, to let him go and just keep on with the work. It was good advice, and my dad has never forgotten it.
Over the years when friends left our lives or the churches we were in, my dad was apt to remind us not to chase them to try to get them to come back. He warned us not to have a bad attitude or talk bitterly about them, but also not to follow them trying to get them back. If it was God's will for them to come back, let Him work it out. And sometimes He certainly did, but other times it truly was for our protection or betterment to have someone removed from our lives.
It may not make sense to you now that someone has been removed from your life, but the Lord has a reason. Let Him be God and rest in His perfect wisdom.
Romans 8:28 And we know that all things work together for good to them that love God, to them who are the called according to his purpose.
Isaiah 55:8-9 For my thoughts are not your thoughts, neither are your ways my ways, saith the LORD. For as the heavens are higher than the earth, so are my ways higher than your ways, and my thoughts than your thoughts.