The answer is, I think, that he's doing it because he can. He looks about him and sees nothing to stop him. Europe was never much of a factor in deterring Soviet and Russian ambitions, but a United States of enormous economic and military power once was. Unfortunately, under the current administration, the U.S. is fading as a power in both respects, and Putin is well-aware of the fact.
He sees us threaten Iran, and do nothing. He sees us threaten Syria, and do nothing. He sees us threaten the perpetrators of the Benghazi attack, and do nothing. He sees us cutting our military and incurring crushing debt, and he rightly calculates that we lack both the clout and certainly the will to contest his moves in Eastern Europe.
In other words, when Putin looked at America under Mr. Bush he, perhaps, saw this:
It's remarkable that the lesson seems so hard to learn, but perhaps Mr. Obama is finally learning it. Weakness invites conflict. The best way to deter people from depredating their neighbors is to send the message that such behavior will be costly, and to be economically and militarily powerful enough to make the threat credible. The best way to prevent war is to be prepared for it.
When muggers know that their potential adversary looks like the top photo they behave circumspectly. When they know that their adversary looks like the bottom photo they hold him in derision.
It is because the U.S. leadership is being held in derision by Mr. Putin that Mr. Obama has had to abandon the golf course to take a tour of Asian nations whose leaders are alarmed that we've lost the will to keep our commitments to them. After watching the President renege on one threat after another in the Middle East, the Taiwanese, South Koreans and Japanese must be very concerned that this is a man who makes improvident promises which he shouldn't have made in the first place, but which, once made, he doesn't keep and who therefore can hardly be counted on to keep promises made by previous administrations.
Who can blame them?