I think the thinking is Russia wants Sebastopol as a navy port on the black sea to counteract Nato --as well as Putin having to act tough in order to have the Russian people on his side. Russia is somewhat nationalistic and the people are loath to see them suffer further deterioration to the image of a relatively strong Russia. It's basically the only card he has to play.

So from a geopolitical standpoint, they are kind of between a rock and a hard place. The west wants to sanction Russia and Russia wants to show it is still a formidable force as well as needs to have a deep water port in the black sea.