That's a dumb instinct we've trained ourselves out of for the most part. At least, those of us that have training - but you know, I go out and stick my hand in dark holes for the fun of it so what would I know?
My timeline is the one where humans are universally afraid to be alone and together at the same. Pack mentality works when you have lots and lots of guns and level headed people, sort of like a redneck filled gas station in the middle of West Virginia where they shoot first and ask questions later.
Otherwise, groups at home get you killed and get you killed fast. Safety in numbers is a lie unless you like shoving people behind you so the zombies get your friends first.