The Point of the Unions

[Kath]

What made Tuor so special that he could become counted among the Elves, when Lúthien and Arwen had to become mortal?

This I thought was to show the theme of sacrifice. A large part of the message of both the Silmarillion and the Lord of the Rings is the overwhelming power of love. The greatest sacrifice that a person could make for love would surely be to give up immortality for the chance to be with the one you love.

As to why it is the women that have to make this sacrifice and not the men I don't know. For a sexist argument toward men rather than women maybe Tolkien felt that women were more likely to have the greatness of heart to make this kind of a sacrifice.