Group celebrations: an "evolutionary urge"?

There’s no end of bunkum just-so stories about the evolution of human behaviors. Not saying they’re all bunkum, just that many are, particularly when they’re in the headline of a news story about politics. So:

We experience emotions socially, says Jack Dovidio, a Yale University social psychologist. Joy by oneself is not the same as joy with other people. Its the idea of sharing it with others that magnifies it.
...
The positive mood in particular makes you expand your boundaries of whos in your group, Dovidio says. So people in positive moods tend to be much more inclusive of what they consider to be in their group.
It's an evolutionary urge, he explains, because being a solidified member of a group was once essential to human survival.

It’s quite true that emotions are social phenomena in humans, as well as individual feelings. But I’m mystified as to why being “much more inclusive of what they consider to be in their group” would have enhanced fitness. Wouldn’t that just make people more vulnerable to exploitation?

I’m also skeptical of the “group cohesion” explanation. Sure, I can imagine how cohesive groups might have had advantages over individuals in the past. It’s a well-worn hypothesis for why any kind of group ceremonies, rituals, or celebrations might be favored by evolution. But as Dovidio points out in the story, there’s no reason to think that large social celebrations give rise to permanent or long-lasting relationships (he mentions post-WWII celebrations as well as the post-Obama Grant Park event). There are plenty of more sedate and deliberative ways that groups can cohere – like, well, foraging together. Other species have long-lasting coalitions without the benefits of giant celebrations.

I can think of an obvious alternative hypothesis: we just want to see what’s going on. And if something big is going on without us, we’re likely to miss out.

Now, granted, that doesn’t explain the people who faint with hysteria when they see Beatle-like celebrities. But then, there’s a lot of psychology going on behind the construction and promotion of these events. They don’t call it “cult of personality” for nothing.