As I said in another comment, the Bucharest summit 2008 started paving the way towards a NATO membership. Until 2014 under Yanukovych Ukraine itself suspended its membership ambitions, in 2014 it continued seeking membership. The Russian response followed and prevented a membership due to Ukraine being in an active conflict and this bought Russian time to either find a diplomatic solution or prepare the invasion.

Taking Crimea and supporting separatists was more than enough to guarantee Ukraine could not join NATO though.

That is a good point, I never thought deeply about that. As far as I know, Ukraine NATO relationships still progressed even after 2014. So even though an actual membership was not possible for essentially technical reasons, the intention was still there and progress was arguably made. So maybe the situation was not judged as satisfactory, no official membership but a defacto membership in the making. Other contributing reasons like resources including water for Crimea.

I agree that Ukraine wanted to join NATO, but that isn't the same thing as that aim being likely to be realised. Putin's calculus is his own to know and I don't think his claims about why he does things can be taken at face value

Why would he lie? And if he lies, what would the alternative reasons be and why would he not want to spell them out?

> 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.