Well, did another invasion and the today's triple champ.
At both my pet went grey to me (was still blue to everyone else, I asked). Checked happiness constantly and was always "Wonderfully". Even after he goes grey and attacks me, he still obeys my commands perfectly. I know for a fact that I'm not accidentally attacking him or anything like that, nor am I personally going grey, causing him to aggro on me for any reason.
It's really weird and a bit annoying. Feeling like I have to do nothing but stealth-lead him around to mobs, instead of also being able to attack them myself, just for fear of my own pet killing me.
I have not yet had a chance to find better +lore gear, but, given that he stays at max happiness and follows my commands perfectly, does it really sound like my taming/lore levels are the problem here? Heck, when I mess with my hellhound pack THEY lose happiness far faster than my WW.