Is not enlightenment the experience of oneness?
That notion is one common reason why some seekers seek. In essence, it is not unlike the reason the addict keeps returning to his/her habit. We got a high and we want the high again.
Someone along the way decided that enlightenment is equal to permanent pleasure; and the pleasure here happens to be a higher state of consciousness.
Given the amount of psychological suffering we can experience the idea of having only the positive half of the universe and discarding the other half can be appealing.
But some inner reflection reveals that to be a pretty naive thing to believe in and pursue. Nevertheless it seems we all fall for it at one time or another in our lives. Whether it is drugs, food, sex, coffee or spiritual practices or rituals, makes no difference as far as the end of suffering is concerned.
The experience of oneness is part of duality therefore it will eventually lead to the opposite experience. The yogi must come down from his cloud and eat dinner and wash the dishes and feel like an animal amidst other animals again. That is what duality is about. Fight it and you will suffer more.
The end of suffering is not permanent pleasure. The end of suffering is the total and unconditional embracing of duality. With the caveat that there is no one to embrace it.