Friends Forever?

I'm currently living away from home for the first time.
Don't get me wrong, I am thoroughly enjoying my "new found freedom" and I have met a lot of wonderful people.
I've also left behind some wonderful people too though, people who have also moved on to start their new lives away from home and now don't seem to have time for any aspects of their old life.
It got me thinking about friends that have passed in and out of my life, very few have remained throughout the majority of my short life which I concluded is because most friends we make are forced upon us due to circumstances; such as friends from school, neighbours or clubs and now friends who I have been forced to live with for the next few years.
Then after those few years will they too, like so many before them, dissapear from my life without us taking a second glance at one another?
I voiced this concern to a friend who I have not known for long, but a good friend who I do not rely upon in a certain environment but who I feel is a friend I was able to choose fully with no other forces playing their part in it.
He wisely told me:
Remember that not all friends that you meet are going to turn into friends for life but that doesn't mean you can't have fun and learn things and take them for what they are while they are around.
I'm not sure how I feel about this still though.
Surely loyalty is a large part of friendship?
How can that exist knowing that as soon as circumstances change again, as soon as we're thrust head first into our "old lives" it will be as though the years we spent away didn't matter and we'll no longer have these new people around?

Maybe it's just my home sickness upsetting me and making me think like this.
I just felt compelled to jot down my thoughts and worries.