[Belegorn]

I cannot say the Elves were selfish when they taught and brought so many gifts to the Númenóreans who became the best sailors in M-E.

"the Eldar came to the Edain and enriched them with knowledge and many gifts" [RotK, p. 352]

They even tried to stay the shadow spreading among the people who longed for an Elvish fate rather than their own, "this we hold to be true, that your home is not here, neither in the Land of Aman nor anywhere within the Circles of the World. And the Doom of Men, that they should depart, was at first a gift of Ilúvatar... some grew willful and proud and would not yield, until life was reft from them... we fear that the shadow arises once more and grows again in your hearts. though you be the Dúnedain, fairest of Men, who escaped the Shadows of old and fought valiantly against it." [Sil, p. 327]

They told the King and his people that they were just visitors in this world, with a fate apart from the Elves, but there were now Men who would hold onto life until they became inept rather than freely give up their lives like Aragorn and the early Kings did. They knew they were not immortal, Elros chose to be mortal, but his later descendants wished too to have the choice to become Elves and regretted Elros' decision.