> They seem to be very unhappy about Japan building up its war fighting capacity.

Much in the same way the same people are upset about Taiwan, the Phillipines, the USA, etc. are building up their military capacity.

Also much in the same way russia is unhappy about Ukraine building up its war fighting capacity.

The unhappiness could come from multiple things, but given China has themselves built up their own war fighting capacity to a much greater degree, it seems the primary motivation is seeking hegemony.