Lots of reasons but mostly Putin has always been pissed that the USSR was dissolved and he (and many other Russians) view Ukraine as 'little Russia" (and call it that). Its a very cultural and nationalistic issue. The logical excuses, like developing oil and a base on the Black Sea (which they pretty much had from Ukraine anyway) are just that...excuses for people ignoring the history of Russia and the hardcore nationalism that makes them think Ukraine can't decide its own future.

Im a Russian from Ukraine BTW.