I think that most of the time, for most people, 'best friends' shift and change as we go through different stages in our lives.
In grade school, I didn't even have a best friend--I was kind of an outcast. In junior high, my best friend was another gal who was probably similarly a social misfit. We ended up going to different high schools, and our respective lives went in very different directions.
When my kids were in school, my "best friend" was a woman around the corner who also had kids in the same school, though her kids were a little older than mine..but nonetheless, we shared many common interests. Once all the kids were out of school, and her husband retired, they sold their house and moved away, nearer to other family. I am in infrequent touch with their daughter via Face Book, and hear from them only in holiday greeting cards. Our interests and activities have diverged.
Then, I was heavily involved in a community theater group, and I had another best friend at that time. Sadly, she has since passed away. I am still in touch, albeit infrequently, with the rest of her family.
Ironically, the only friend I still hear from via e-mail in addition to holiday cards, is my old friend from junior high...and she lives about 200 miles from me, so we don't see each other, or even talk on the phone--our lives and interests have drifted apart--yet, we stay in some contact after all these years.
People change; lives change; circumstances and interests change, and it would be rare to have one "BFF" throughout life...because "BFF" stands for 'Best Friends Forever,' and nothing is forever.
My current best friend is my husband, and that's as it should be.