Having less testosterone. Testosterone makes us horny as hell, while they have estrogens which makes them emotional.
So the average girl wants to feel that someone loves her. Sex is the currency to get this feeling, and the harder it is, the more valuable that feeling is for her. If you give her this feeling for free, then you won't have the sex she was willing to give you, and you will be friendzoned. If she percives that you have more demand (other girls are interested in 'buying with sex your love'), then she will offer more sex to get that feeling. That's the reason you have to focus on your selfimprovement (to increase your demand) and the reason she has to see you with other girls. Think in terms of offer and demand.
In resume, girl feels attracted to you when she percives you are valuable, this is when she percives that you are an alpha, and that's the reason she is willing to fuck with you, to get you to love her (that's the reason that if you fuck her and dump her she feels used). Also not being a dependent being increases your alpha status.
But beware, the average woman will do whatever its in her hand to make you a beta and cuck you once you love her. Then she will try to get another alpha. This doesn't makes her feel happy, but it's programmed into her DNA. Also they love dramatic events.
>Pro tip: If you want to get laid easy with a girl just tell her you want to fuck her with another guy (that you know is a cuck and won't do it). Say it with confidence, looking into his eyes and with a half smile. She will percive you as self-confident, because you don't care to share her with the other guy. This will trigger a sentiment of willing you to want her