When we first meet someone we want to become friends with we stay on our best behavior and try to show this future friend our best sides, so as not to scare them away with our faults.
But by the time this person becomes our best friend they've seen many, if not all, of our sides- the good, the bad, and the ugly.
If you're wondering whether someone is your friend or your best friend consult this comic by Jacob Andrews and decide which side seems similar to the way you and your friend interact.
-Via CollegeHumor