What surprises me is that there is so much written about flirting, courting, having a successful relationship or marriage, and so on, and so little about friendship (I am referring to things of value. Otherwise, plenty of trash around).
On one side, everybody wishes he/she had more friends. On the other, it seems so hard to keep friendships, to nurture them, to let them survive moves, changes of habit, etc.
Also, after moving, it seems that I am always the one that needs to search for my friends, to call them, to email them, to tell them that I'm back and want to meet. Very few of them, and not often, want to know how's life in Luxembourg, in Singapore, in San Francisco, or watch pictures, or listen to me telling them stories.
Strange. After a while, you start thinking "I shouldn't be putting so much effort to keep it alive. Let it be.", and then they start fading away. Although, I agree with you, the strongest ones really stay pretty much the same through the years...
Unless you change too much, which is what I believe is happening to me, and might happen to people that travel and move too much.