The fading friends

I like to meet up with old friends, the ones you once had a good time with and then faded away along the years. My really good friends I can count on one hand. However, I remember in high school, when I was about fifteen years old, I thought all my friends would last forever and I would always keep in contact with them, no matter what. Now, five years later, I'm left with some of my friends from high school, but the ones I consider the most valuable, I have met only in the last three years. 

To provide you guys a little background, my studies are not the average studies you would expect. I am doing a bachelor degree in European Business Management and Marketing, which takes three years to finish. However, I study those three years in three different countries thus three different universities. My first year - in Denmark - I met the friends I was talking about that I'm sure I will remember for all my life and I will have contact with them for a long period of time after college will end for me. You know, I like to call those friends 'the wedding friends'. The ones you really want to be there when you get married. There are about four of them, which I consider as enough, since I have about the same amount of friends from high school that I can call 'wedding friends'. 

The point of this post is: 
how come friends you once were so close to, can fade away within a year of seperation? 
Of course, you grow apart a little. You may go to different colleges or universities. You may choose a different country to live in. You may even choose to completely turn your life around. People change, but can they change so much that contacts once so tight can turn into a long forgotten memory? Is it possible that by the age of fifty, we are nothing like we used to be when we were in our twenties? Is there nothing of our youthfulness that will stay when growing up, when aging, when building our lives? 

It scares me that already now, with only twenty years old, I feel like my life is very different from when I was fifteen. Every single decision you make changes the path your life will take in the future. That's frightening. I guess that's why someone invented vodka.