Russia and its predecessors have a history of about 800 years of being invaded by one group or another. These included the Mongols in 1223 (who weren't driven out completely until 1480), the Crimean Tatars in 1571, the Polish-Muscovite War from 1605-1618, the Cossack uprising and incursion from 1667-1670, Napoleon's invasion in 1812, Japan in 1904 (mostly naval, but still an attack by an outside power), and Germany in 1941. The collapse of the Warsaw Pact and the eventual joining of NATO by every non-Soviet member (plus the former Soviet Baltic states) is often seen internally as an incursion into Russian interests, with the discussions of Georgia and especially Ukraine joining NATO leaving Moscow surrounded by enemies who are only a couple of weeks' fighting from the gates of Moscow.
It has a traditional reason to be xenophobic, regardless of whether it makes logical sense to those outside Russia. The Warsaw Pact nations and the former Soviet republics were buffer zones for Russia, land they could afford to lose, at least temporarily, while ensuring that Russia itself survived.
The current situation is only barely tolerable to Moscow, and is exacerbated by recent low Russian birth rates and low life expectancy, leading to a decline in population for nearly two decades. While this is turning around recently, the increasing birth rate is also heavily subsidized by the government (families who have more than one child get a lump-sum payment of about 428,000 rubles (worth about $6800 now, as much as $11,000 before the downturn in oil prices) and heavily dependent on the economy, which is in a difficult position, to say the least. As fragile as it is, it may decrease due to hits to the economy if more sanctions are added or the nuclear deal with Iran induces further oil price reductions, and perhaps in that case by increased alcoholism (more than a third of deaths in Russia are linked to alcohol).
I don't expect Russia to go to nuclear war, but they see the situation as desperate, possibly bordering on disastrous, and it puts them in a difficult position where even pie-in-the-sky ideas (like a Bering Straits bridge, to get back to the original post) sound like a good idea. You might think their position self-made, illogical, or even stupid, but it's very real. You don't have to agree with it to understand it, but dismissing it is just dangerous.