This question has bugged me for years. Let me put it into perspective for you. There's two scenario's I would use as examples.
The first, some kind of intelligence, whether it be natural or born of technology, begins to assimilate humanity. Once you're taken into this Hivemind, who you are no longer exists, you are simply a part of something larger, a blood cell to a greater body. You no longer have desires, fears, needs, passions, you're simple existence is utterly focused towards the common goal of the collective.
The second, some kind of creature, person, entity, whatever, takes your resolve through mind control, natural chemicals, possession, whatever. Once they have you under their spell, you are completely submitted, and the simple act of submission brings with it ecstasy you could never feel otherwise. A word, a touch, their simple presence is all you need, and you would die happily at their order.
Both scenario's, you give up the self for a much different way of life, a difference existence. So the question is, how much do you value your 'humanity' for lack of a better term. Could you do without the dynamics of free will and just submit to an easier state of life?