The main need for this comes from the rise of China. Specifically, its rise in economic power and military capabilities make is so that 1) the US is no longer able to deter both Russia and China combined, but needs to prioritize China (its main geopolitical rival) and 2) Russia can openly engage in armed conflict without the fear of Western sanctions, as China will keep Russia's economy afloat (mainly via purchase of energy).

With Trump back in the White House there is an additional motivation for EU countries to up their military spending; at best the US is now viewed as a less reliable ally (that may or may not help in the face of conflict), at worst the US will align itself with autocratic regimes to divide up the world in their respective spheres of influence and actively undermine the EU.

So if you accept the premise that democracy (and the liberal, rule-based world order) is worthwhile defending, increased military spending of EU countries is in fact a "good thing" (as in, better than the alternative of submitting to imperialistic authoritarian neighbors).