Some friendships are borne of nature. In a roomful of adults, two children will find each other. As will two adults, in a roomful of children. Like recognizes like. These are magnetic, mellifluent, and mysterious. When the likeness is overt, the friendship is easily understood by all. Some can even see them being friends before they've met. When the bond is covert, the pairing can be unexpected and baffling from the outside, and even stronger and deeper on the inside, for it shares secrets unseen and unknown, except by the two. They are friends because they can be no other way.
Some friendships are borne of time. Sitting together in 3rd grade, on the same team in high school, shared a commute in college, or a mentor in grad school. Neighbors, office buddies, room mates, project partners, professional rivals. All folks who never would've met but for their allocation in the cubicles of capitalist civilization. But even of these, those with natural affinity link deeper and last longer. They are friends because they have been.
Some friendships are borne of circumstance. The doctor who delivered your baby, the stranger who caught you from falling on vacation, the acquaintance who was able to recover documents and photos from your crashed computer, the one person who talks to you at a busy party where you don't quite fit in. Moments from your life where a simple generous gesture by one good person saved you, in the largest and smallest of ways, where you can feel nothing but gratitude for an unexpected kindness. These friendships may not be kept up as much as the others, but they are remembered.
It is good to have friends. It is lucky to have friends. The better friends come from more than one of the above, and remind you of the fondness of friendship. But the very best friends, from wherever they come, will come for you, regardless of their natural fit, be there time or not, in circumstances well and ill. And they remind you of the fierceness of friendship.