Sauron mingled among the elves and decieved them. Sauron also a maiar that was corrupted by Morgoth.
Quote:
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It was Eregion that the counsels of Sauron were most gladly received, for in that land the Noldor desired ever to increase the skil and the subtlety of their works. Pg 287 Silmarillion- Of the rings of poer and the third age
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He was in disguise and only a few of the elves doubting him( Elrond and Gil-galad). He took on a name, meaning Lord of Gifts and offered to help and advise the elves. Sauron wanted the elves for their power. They were also harder to sway to teh "dark" side unlike the men. So the elves were greatly desired by Sauron.
That is atleast the start and what is offered in the Silmarillion( which I trust a little more then later stuff that has been put out).