I don't understand why the speaker says "love me for loves sake only."
In order for someone to love someone else there must be certain features or characteristics about the person that the other finds appealing. Otherwise, if the person loves the other for "loves sake only" then he is able to love any person as his love is not limited to his lover's physical features or characteristics. It therefore doesn't make sense because the love is then not eternal seeing as it is not limited to a specific person but only to love.
I just read through this poem several times and...I think I agree with you. While literature brings us the wisdom of the world, we must recall that it is human wisdom and prone to error. She seems to be saying, "I want you to be in love with the idea of being in love," which (as you say) leaves her adorer's love open to anyone.
She clearly wants his love to transcend anything that is liable to change--not just her looks, but the things she says--but logically, her suggestion to "love for love's sake only" doesn't work. It ultimately has no hook upon which to hang.
Then again, some people can have faith and believe for the sake of believing, so why do we find it so improbable that a person can love for the sake of loving? And what are we thinking, anyway, trying to be logical about love?