Reason number one: a diplomatic solution is almost impossible at this point.
Reason number two: Putin is under heavy pressure domestically to do something to protect the Russian speakers of east Ukraine.
Reason number three: If Kiev is able to regroup and lay siege to Donetsk this will be an extremely brutal confrontation.
Reason number four: Russia is not likely to stand by and watch Kiev lay siege to Donetsk without taking action.
Reason five: If this situation continues to go south, Washington may double down and push the envelope even farther in terms of covert operations.