Is it bad for a 14 year old boy and girl (really close friends) to know literally everything about each other?

They don’t know literally everything about each other. I’m assuming this is hyperbole on your part.

They may be BFFs and friends since kindergarten. They may hug often and tell each other secrets repeatedly.

They may be the closest of close friends.

But they don’t know everything about each other.

At fourteen, what they know and what they hope to know will change rapidly.

Even adults that have been adults for a very long time don’t know everything about their close friends.

If there’s something that someone doesn’t want to deal with or can’t, they’ll often repress or hide it, even from themselves. You, as a close friend, may suspect something’s amiss, but you certainly won’t know the specific motivations behind that.

Sometimes people just need distance, even from very good friends. I don’t know about you, but I have very definite boundaries, even for those I’m close to. I’m also a survivor of emotional incest. So when a platonic friend, casual acquaintance, or family member pressures me into caring for them like an unpaid therapist, a parent, or a lover, I usually retreat in disgust.

14-year olds are rapidly developing into their adult selves. What constitutes that friendship will change and transform as well. The friendship may even end, even if they don’t want it to. This is commonplace.

Boundaries are healthy. I can’t do everything for besties and they can’t do everything for me. We shouldn’t want to, anyways.