What determines when a friend becomes a best friend?

This is how I behave around my BFF:
– I insult her. She never gets angry and insults me back. We have a good laugh.
– She doesn’t like some of my hobbies and neither do I to some of hers, but none of us express our distaste. I can happily wait for her for an hour so that she can finish her make-up, and she happily goes with me to buy a “dark” book for my collection.
– Any secret can be shared without fear of being exposed.
– I feel an urge to help her immediately whenever she is in trouble, no matter who causes the trouble in the first place.
– We touch each other frequently.

Sorry. There are no guidelines.

There are people 80 years old that met their best friend in kindergarten.
There are people of every age that have a different BF every day, or may change on the hour, minute.
A friend becomes a “best” friend when you believe you don’t have a friend who knows you better.
The better you know yourself, the longer the BF is likely to last.

I don’t think I could begin to explain when/why/how it happens, it just does.

It is like love without the sex. It just happens. You can’t define it. You just know when it happens.

Trust, respect and understanding.

From my perspective it’s someone who doesn’t hang around everyday but you know he’s there and when you’re in trouble he’s the first one to come and you wanna turn to

I believe best friends are for kids only, a wide circle of very good friends shouldn’t be distinguished by who is the most bestest in the whole wide world.
That was kiddie talk, coz that what it amounts to.

When I can fart around them and they laugh with me

Seeing you at your lowest without judging you but their friendship remains solid.