What's so special about childhood friends?

I take time to make friends. I take time to get to know a person, trust them and truly be their friend. But I seem to instantly break all my rules when I meet a childhood friend. I just love and trust them even though not have seen them for years. Its really wierd and it made me wonder why. Is it that i met them at a time of my life when all my defences was down. Is it that I met them when they too were, innocent like children? but the best part of it all is that I am able to love my childhood friends the way they are. I accept them in spite of all their faults. They're like family. I'm so glad that I have some great childhood friends who I share great relationships with.