Twins

My best friends are twins, but they aren't related.

They are from different families, and each has a twin brother.

I know all 4 twins: we went to school and played sports together.
But, I'm only good friends with one of each set.

I'm not sure why?

It's not because they are identical and I don't like being confused.

It's not because they didn't like hanging out with their twin, and I had to make a choice.

It's not because one got married, so I ended up becoming closer to the single twin.

It's not because one twin moved away, and our friendship ended because of the distance.

I guess it was a personality thing:

My friends' twin brothers are pricks.