1. Everything that exists is part of a bigger whole. You are also part of that whole. Infact, you arose out of that oneness and so cannot be separate from it.
2. Your physical body cannot exist for even one second without the environment around it. Take away air, and you're finished. Take away space and you're no longer there. Take away everything your senses detect, and you're as good as dead.
3. If you were separate from onenenss, that would mean that the oneness isn't oneness! The very fact that oneness exists, would mean it includes everything.
4. Oneness couldn't be something to be attained. For what would you experience if you got oneness? All experience imply a duality of experiencer and experience. You can only experience the world through duality. The duality comes from the onenenss, and is perfect just as it is.
BONUS REASON!!! :)
5. Oneness couldn't be attained in the future, because the future never comes. You always have been and always will be in the present moment. So this moment now, is onenness. This is it. You have arrived and always have arrived.