About friends who leave for no reason, who don't come back, or even are hostile, I would say not to worry too much: they were not real friends. However, I do agree that it hurts a lot. When I was around 15-16 years old, I lost almost all my friends over 1 year, without even knowing why. To be honnest, I had never had a lot of friends anyway at this moment, but many children who were "neutral" toward me even started to be hostile, while the ones who were friendly started to be hostile too. This year was difficult for me, as I didn't understand what was going on. In fact, even today I am not exactly sure, it might come from the fact that I was weird (and still am by moments). But in this case, why at this moment and not sooner?

The real sign that someone has become a fanatic is that he completely loses his sense of humor about some important facet of his life. When humor goes, it means he's lost his perspective.
Wedge Antilles
Star Wars - Exile