IMHO the reason was many

1:  A general feeling of NATO as possibly attacking russia and russia was in no position to defend itself with such a huge amount of NATO entry  positions

2: Russia wanted the oil and gas located in the Black Sea off of Crimea shores, thus Crimea had to be a solid russian territory

3: Russia needed the warm water port and nobody to wage war over the ports

4: If russia could gain contrl of all of Ukraine then additional oil fields and of coarse the grain exports would be under russian control

5:  Russia was tired of paying Ukraine billions every yr for tolls to use pipelines running through Ukraine caring gas and oil to European markets

6 Putin wanted to restore the country as the old USSR that he knew in earlier days before the break up of the Soviet method

So you see, according to Putin this was more than just a land grab