Imagine you could only have one friend for the rest of your life. If you don't find one then you will be left alone.

For many, that's a important decision, so they are always attentive to potential friend candidates. You don't want to humiliate yourself in front of them, so you shy away. You want to show them that you are good friend candidates, you try to show off. Someone else is a good friend candidates, you feel insecure. Someone gets angry at you because you were apparently showing off without realising it and feel threatened.

Its a competition that your body compels you to participated, and it gets exhausting at time.

Now add "and make babies", remove being able to verbalize this, and you got sexual desire.

It should be noted do, intensity varies from person to person. Those that have it set too high cause problems to themselves and others.

I can give you a little insight. I'm not sure if these comparisons will hit home or not, but I'll try.

If you order food somewhere and the server brings you something extra, or a mistake is made and they let you keep it....do you feel a small, tiny swell of specialness?

When you make a joke to a group of friends and one of them really is enthusiastic about it and laughs a whole lot or tells you that it was a really funny joke to them, do you feel a warmness and or specialness?

Do you feel a secret little spike of maybe I'm a lil bit more special than these people around me right now?

If this kinda rings a bell for you, try amplifying mentally by like a hundred fold...that specialness with a person you are sexually compatible with seems flirty if they appear to be treating you special. If it is true or not doesn't really affect the feeling that springs up in your soul. That's the clouded judgement. It's like chasing a dragon when you feel that someone thinks your special. That's a high feeling.

Fwiw, my fiance was my best friend before we were together. She happened to be on the same wavelength as me when it all was going down.

I've had a few friendships with this tension, some became relationships, in my experience some issues can be changing behaviors because I don't want them to think I'm trying to get with them, like fewer compliments when I would others, or there can be a subtle sort of jealousy if they are interested in someone else. Or vice-versa, I might think things they do are signs of attraction and not friendship and so feel mixed about reciprocating unless I want to pursue a relationship.

It gets in the way by you having great times with your bestie, joking around, being open and chill. Then something, maybe one of you, maybe something external, triggers the old lizard brain. "Hey", your brain goes. "You know, this is great. They are great. This would be a great person for reproduction and this a great time for the horizontal fandango!". But this is your bestie, this is not why you're here. Your conscious brain does not want to do the sexy sex with them but your primal urges do. You're in a state of conflict in your brain, trying to rid yourself from one of those essential parts of your very being. What is worse, they have either triggered on the same thing or picked up something subconsciously and they are in the same state. Instead of being all relaxed and fun and open, you are both self conscious and stiff and trying to play it off. And you hate every second of it.