cause we are fully human and they behave like a breeding animan in relationships. while you struggle with lust love and companionship she intiutively strives to find the best posibble partner.
she will use you to get things done, to fee as if shel has a dominant male in her worst case scenario to feel as if she has managed to scape off a male.
she will never be close to you only because you're her partner I mean she may be comunicative towards you but it will never be the same sort of closeness one as a human has with fellow friend.
If you're not enough of a man in that intuitive part of their brain she will withhold sex from you and then possibly make you a cuckold.
And many more things having their root in that in that they possess an animalistic instinc towards relationships if they fall in love with you I acomplished this once and I could do it again many times but won't but I have further confirmed it with tellings of female friends of various ages, if the fall in love with you in the proper romantic sense it toasts their brains and they sabotage it, I don't know wich evolutionary function that serves.
The difference betwween us and the average man is being happy kowing all that and being sad over the emptiness it brings or being happy playing love with a being that plays the breedind without you knowing it.
there you have it.