There are at least two issues here. One is that Russia has repeatedly suffered invasions* so they're f'ing paranoid. The other is that having nukes *should* make them a lot less paranoid (who's going to invade when that means their homeland gets nuked?) - but that doesn't seem to have happened.

*They're in a large flat area with barely-usable (but ginormous) farming acerage. The flat means any army can rumble through. Poor ag production means a small (relative to area) population - so they need to invade outwards until they reach geographic barriers that they can hold against invading armies.

There's a lot there. If you're interested, check some of Peter Zeihan's books (or youtube channel). He used to run the analysis department for George Friedman at Stratfor back when George ran that. Peter's been doing this on his own for at least a decade now
I don't 100% agree with either of them, but they offer useful frameworks for understanding geopolitics.