People aren’t upset that others are running in stacked groups. They’re upset because they’re being punished for not running in stacked groups themselves. Sure, they could do the same thing and see what a huge difference it almost always makes. They could all be social, fingerprint some rainbows and hold hands, chat and play with all the toxic a-holes, but yeah, not exactly what a lot of solo players want to do. So basically, it isn’t that solo players resent those who group up: its more like they resent the game for being designed to punish them and nudge them into grouping up as well.