I agree with Derrick. People move interstate or overseas and lose contact. However there are many people in this world who are still friends with the people they met in primary school.

Unsinkable like the Titanic? Friendship forever is a great concept but people change, circumstances change and even with the smaller world created by the internet when friends move away it is easy to forget about them after time. "Out of sight out of mind". How many people do you know are divorced? In America over 50% of marriages end up that way. I'm sure when those folks got married they were thinking "unsinkable". Not pessimistic, realistic. Strive to have many good friends but understand you cannot control them or their circumstances.

There's also the very real possibility that you (whomever) form friendship with people sometimes, move away or lose touch, and years later, can pick up right where you left off, because you were close enough with each other to begin with. On the other hand, sometimes things happen to come between friends and their friendship which damage it irreparably. Always depends on circumstances. :-)