Best Friends are old ones

Why are your best friends often the ones you made when you were a kid/adolesent? Sometimes I wonder, as adults we give, in the expectation to take back. Most our relationships are based on this solemn fact. Which, as somebody that was facinated by economic moddeling of human tranasctions, I find this quit good. In a sense that it consits perfectly with the 'rationality' assumption of people.

But how come the childhood relationships, that don't have that much of a tarde sense into them, last better than the ones built on the explainable patterns?