This is incredibly naive. Hunter gatherer communities, especially those in regions without an abundance of food, are and were extremely selective about who were accepted and who weren't. This starts from infancy where non-desirable babies were simply killed. Estimates vary greatly but perhaps around a third to half of "modern" hunter gatherer tribes practice infanticide. The stated reasoning behind infanticides is often extremely vicious and comes down to "he/she is not a good fit for the tribe", or in other words "nobody likes him/her". This fact alone might be one of the major explanations of the high rate of prehistoric infant mortality.

But if you are even allowed to grow up and become an individual, things might be somewhat better once you are part of the in-group, but that does not factor in the fact that human empathy has an overall tendency to switch off if you're not. Even if you're loved because you're kin, your neighboring tribe might still kill you, or you and your kin might kill them, for entirely petty or cynical reasons. The prehistoric bone record supports this as well, seemingly human-weapon related reasons is the most common cause of death.

You can also examine your own emotions to get some idea of our evolutionary environment. Loneliness hurts, to the point where it has measurable negative health impacts equivalent to smoking a pack of cigarettes each day. Your brain is screaming at you not to be lonely, but why? Well, in our ancestral environment, being excluded from the social group meant death, so most individuals that did not have a profound and visceral fear of that happening got their genes consistently removed from the gene pool. For loneliness to be that big of deal, being excluded must have been an easily available option. If everyone loved and accepted everyone unconditionally, this emotional state would simply not have evolved.

Humans quickly become extremely brutal once the environment necessitates it, up to and including cannibalizing your own kin. Infanticide and murder of both ingroups and outgroups is historically commonplace because it was also commonplace prehistorically. Even modern tribes, that live in relative abundance, are still brutal in many ways to this very day.

But of course, when you look at any group of individuals in a tribe survivorship bias will dictate that it all looks nice and rosy. But you might want to check the skeletons in the cave before you pick that as your conclusion.