What do we have on how Beren and Lúthien are attracted to each other? Lúthien, who could have had anyone she wanted, goes for someone who her father would disapprove of right off the bat. I really started to think about this after a couple times watching Julian Fellowes costume dramas, where a number of romances are unequal pairings owing to social class.
You have few marriages across social class barriers in Tolkien. You can have some across the "species" barrier, but both partners are their respective class in their respective community.
The only exception is - as far I remember - the Mariner's Wife and this marriage starts the development that brings about the downfall of Numenor.
Remember that Luthien is an elf and as such she has no "life-goals" except simply being alive - being herself - which she did succesfully for millenia already. She has no pressure to acomplish or to become something "before she dies" because she is lacking exactly this very "dead-line". As such they cannot really bond over shared "life-goals".
While humans struggle to become what they should be (or "are meant to be") elves start out there already and their only struggle is to stay true to what they already were.
Had Luthien had any ambition to become more than she is, this would be suspect - this exactly how Melkor and Mairon or Maeglin started out. For her it can really be simply infatuation (born of curiosity and a genuine fascination with the freshness of "something else" than herself) as elves do not get bored easily and are usualy constant in their feelings once they have committed - or something is very wrong with them.
From Beren's side it is a little more complicated - true, Luthien is mesmerising and ever-young and has no mortal woman's failings. Still humans do get bored with anything after a while - even with perfection.
Also Beren's motivation seems to be in big part to prove his own male worth to Thingol and to defend his own honour and that of his house. And Beren does not really live long enough after first meeting Luthien for his infatuation really to start to wear of as yet (merely for 2 years from when he meets her to when he dies the 1st time - which is short enough for a human to be still in the infatuation state).
And afterwards he gets reembodied and forbidden from meeting any other mortal - so he is no longer has any alternatives there - as no other elleth would take him anyway - so in whatever direction his feelings might have developed, he has to stick with it - which he also is honour-bond to do.
[My aversion and opposition to forcing elves into a human-like life-goals' framework has been only strengthened by the effect it recently had in RoP. If you want to force Luthien into it, then do it please after her reembodiment - there at least it would make some sense.)