It's a reasonable fear, but I wouldn't allow it to be something you dwell on. Inevitably, we do lose a lot of the people we know over time, simply from drifting apart. However, that doesn't take away the positive memories you have of them. Those are yours to keep, reflect on, and cherish. And for every one person that moves on to a different path, there's one more person in the world that might bump into yours. Life is a long journey, and even though the people we take that journey with may come and go, you'll often only be alone if you so choose.