> what would the alternative reasons be and why would he not want to spell them out?

He has spelled it out before--he believes that Ukrainians are really Russians, and therefore Ukraine should not exist as a separate country because why divide a nation in two.

I had this discussion a few days ago and I read the entire essay Putin wrote - if this is the best evidence for the reuniting a nation explanation, then I am not convinced at all. Compared to decades of opposition against NATO expansion this is laughable evidence. Go, read it [1] if you have not.

[1] http://www.en.kremlin.ru/misc/66182

Is this a serious question? Politicians lie all the time! Why? To get support, or to minimize resistance. And Putin manipulates and lies very well.