Once, when I was small, a horrible rumour went around school that my best friend and I had "broken up". I remember my heart sank - did that mean she'd promised to be best friends with someone else and not told me? Luckily, a quick meeting ensued and our best friend-ness was reaffirmed - it was nothing but gossip after all! My memory's quite foggy, but I'm quite sure we were the longest running pair of best friends in our year, and a bit famous for it too.
My point: as children, our friendships are everything. They can feel like mini platonic marriages, or - outside of familial bonds - the anchor to our miniature world. Maybe that's why kids form friendships with such speed and ease; for a while there it's almost the whole point of life. (Plus we're a lot less fussy. I'm sure if I re-met half my childhood friends, we wouldn't have much to discuss. Except that time the school genius started screaming about the planetary system and tried to kill us all with a broom.)
Anyway, then you grow up, get lumped with adult obligations, not enough hours in the day, etc. And the energy directed at forming and maintaining new friendships is dramatically diminished. As are the friend-making opportunities: no more lolling around on the mat for you, munching apples and burbling songs with your peers. (I would assume.)
Still, by adulthood, most of us have made enough friends along the way to feel content - and have kept some semblance of the ability we had, as children, to form new friendships. I know a couple of my closest friends were only made in the last year or so, which gives me hope.
But what if your chum-number just isn't doing it for you? You're sick of your old ones, you've just moved cities, or your friends have all moved to London to battle the recession?