I believe he's also doing this for reasons that are outside of his "NATO bad, eastward expansion bad" shtick, that he's constantly on about. If I understand correctly, he believes that Russia has a fundamental right to Ukraine and believes that the Ukrainians are essentially "one in the same" with Russians from sharing a common language to various Eastern values. I think he wants to "save" Ukraine from the "corrruption" of Western influence. I could be totally wrong, but I kind of got that vibe when he was rambling on earlier during his speech before declaring Russia's "recognition" of the "independence" of the DPR/LPR occupied terrortories in Ukraine.