For JRR Tolkien, "elvishness" is a quality that first entered Mankind by heredity from the rare "interbreeding" of Men with Elves; especially Beren and Luthien, Tuor and Idril and Aragorn and Arwen - but perhaps also from one or more unions of Men with Silvan (i.e. "lower") elves such as happened in Dol Amroth.
After which, the "half-elven" strain was transmitted in part by a kind of heredity (which is not strictly genetic); and also by close-association-with and even the love of elves - via the phenomenon of "Elf Friends".
Indeed, this second "associational" rather then hereditary elvishness seems likely to have been the most important; since it seems very unlikely that all the Numenoreans (for instance) were actual descendants of Beren, Luthien, Tuor or Idril - most seem to have been members of three particular Elf Friend tribes or clans.
I find it fascinating to consider what spiritual realities lie behind Tolkien's elves, and their relationship with Men; and my speculations have recently been fuelled by reading-though a fascinating and wide-ranging book called Red Tree, White Tree by Wendy Berg - who was a disciple of Gareth Knight; later (with her husband) becoming leader of the Gareth Knight Christian magical group, after GK retired.
Berg puts forward many and various speculations on the subject of "faeries" including a detailed consideration of Tolkien's elves; also folklore, and the whole range of Arthurian literature - which she interprets as being, ultimately, "about" faery/ Men relationships.
In particular, she regards faeries and Men as two distinguished-sides of an original-whole; and of the two sub-species of "human" as having been sundered in the remote past - and with, therefore, the long-term implicit goal of becoming re-unified - despite the many problems such a destiny will entail.
(Problems such as are, for Berg, the principal subject of Arthurian romance.)
My thought was that elves and Men can be distinguished in terms of motivation.
Elves/ faeries value and are more immersed in creation, and therefore live in harmony with the natural world; but, on the other hand, elves tend to become passive, contemplative; and instead of contributing to the world, they tend to try and preserve the past, and to stay the workings of time.
Men are more self-conscious of their distinction from nature, and selfishly tend to impose their will on creation and respond to temporary impulses; yet this heightened self-consciousness and distinction from nature is also the basis of freedom, and can motivate Men to contribute personally to divine creation.
In Owen Barfield's terms - Elves/ faeries are prone to yearn for the immersive unconsciousness of "Original Participation"; while Men are prone to the alienation and despair of the "Consciousness Soul".
And the hope for a re-uniting of faery and Men; is the hope of combining the best of both: combining the free creativity of Men with the harmony-with-divine-creation characteristic of elves.
Tolkien regarded elves and Men as both "human"; yet as separate creations, and therefore eternally distinct. Berg takes the different stance of understanding elves and Men to have been two directions in which humanity grew after an original unity in the Garden of Eden.
However we may model or literalize the explanation; there does seem to be a true insight and wisdom in this business.
It seems probable to me that this ideal harmony and union cannot be achieved - except temporarily and in a limited fashion - in this mortal life and on this earth.
Yet the distinction of motivations has validity as a way of conceptualizing the ideal state of both elves and Men towards which we may aspire.
And also a warning - on both sides - of the hazards of a divided and partial consciousness.