Why do men think we are in love with them after 1 date?

Men overestimate their desirability (as demonstrated by this study), and on top of that, have been conditioned by society into thinking every woman is just begging to be married off and impregnated by them and should be grateful for even being considered for the position of their maid and surrogate mother.

It's annoying, but don't waste your energy proving to them you don't like them that much. They'll think its a case of 'the lady doth protest too much'. Time will tell who likes who more. If anything, it can be used to your advantage - if they convince themselves that you like them so much, then if you ever go cold on them because they step over the line or underperform, they'll work harder to get back to a place where they think you like them more. That psychological reaction is actually why narcs lovebomb you and pretend to like you more at the start, then do a 180 to make you fight for their approval. Of course, in this instance, you're not doing anything other than being yourself and letting them think what they want, instead of actively manipulating. I am very pro using their egos against them though, lol.

Also, for the above reason, this is why you should be wary of men who DO seem to like you so much more at the start. In fact, be wary of all men until they give you substantial reason not to be over a prolonged period of time.