Belarus has basically been turned into a vassal state. Lukashenko was propped up and the election stolen on his behalf by Putin, just like Yanukovych before he got ousted. Putin's preferred method of operation is installing puppet governments that he more or less has complete control of. Lukashenko is literally one of his lap dogs, he was a member of the Communist Party of the Soviet Union all through the 80's and up until the collapse and is "Independent" now. Putin serves as a Chairman of the Council of Ministers for which Lukashenko is Chairman of the Supreme State Council of the Union State.
Putin also prefers to have "buffer states" between him and NATO countries, hence his Chairman status on Belarus' Council of Ministers. It keeps him close to his puppet.
Currently he has control of Georgia and Belarus. That leaves Ukraine, Estonia, Latvia, Lithuania, and he probably wants Moldova back as well.
Ukraine is the first step after Belarus because Ukraine is essential to unfettered access to a year-long sea port, which Ukraine gave him access to previously. His only other ports capable of shipping out of such as Arkhangelsk are frozen in for significant portions of the year. The REAL reason for the annexation of Crimea in 2014 was to maintain hold of Sevastopol, his only green water sea port that isn't inaccessible for parts of the year and happens to have a Naval base stationed there that is of vital importance to him because the Russian Navy doesn't have the capability of leaving pelagic zone water.
That's right folks, the once great Navy of the USSR is now so pathetic it can't leave continental-shelf depth water and couldn't circumnavigate the globe or come anywhere near threating the Americas, Australia, New Zealand, Madagascar, or even Indonesia, Papua New Guinea, Philippines, he can barely even get to Taiwan. Which is why he's so desperate to reclaim Ukraine because right now the only access he has to Sevastopol is through his non-freight capable port in St. Petersburg or his non year-round port in Arkhangelsk. So he wants a land bridge to Sevastopol back very badly.
The reason Lukashenko is going along? Well, he'll end up replaced if he doesn't. He'll just simply disappear. He's been a Soviet all his life and he's still a party man, through and through. And when Putin is jonesing for something that bad, he'll kill half the party to get it if he has to.