1. The sheer desire to dominate. Really it's always a factor for any policy, the question is when it's becoming seductive enough.
2. Putin, being mafia-style boss, needs certain balance of force inside Russia. Professional Armed Force means too much risk of military coup, while Soviet-style colossal conscript Armed Forces means too much risk of revolution in the case of mobilization, so Putin needs neither. His obvious solution is to have an Armed Forces with very sloppy trained soldiers and very inflexible, anti-intellectual, corrupt and fanfaron officer corps - so that they are constituting as little menace to him personally as it's possible. To make such an army - you have to eliminate the most brave and responsible of these men in Armed Forces. To send them die in some ill-prepared war is a good solution.

Emperor Putin does not have a LW account, so he posted his answer in a book.

A: Because Ukraine was shelling Donbas.

B: Because Ukraine was threatening to invade and conquer Crimea.

C: Because Ukraine was developing/threatening to develop weapons of mass destruction.

D: Because Russia is convinced that the west is out to get it, and the Russian people desire victory over the west, serving as a show of force and thus potential deterrent to future hostile actions.

E: Because Ukraine cut off Crimea's water supply, and other such nettling actions.