Lots of people agree the internet is full of outrage content these days. I think this is so obvious I'm not even going to post sources that say it.

Why this is the case isn't completely clear. I count at least four common theories which of course aren't exclusive:

media companies are shrinking, so they're desperate for clicks and have stumbled on outrage as a good click getter

it is Twitter's fault, the structure there incentivizes outrage

increasing political polarization has infected the internet

coddled youth, used to carefully curated experiences, can't handle disagreement like they should

All of these might be contributing factors, but I want to point out another that I haven't seen mentioned.

The strength of an outrage response depends not just on how outrageous an act is, but also on other factors, how recent it is for example, and how close we are to the victim that something outrageous is done to. And in particular, it also depends on how close the outrageous person (the perpetrator) is.

Imagined examples, where we hold the act, victim and recentness constant and vary only the closeness of the perpetrator:

If somebody else's child steals from a shop I don't like it. But if my own child steals the same thing from the same shop I get really angry.

When the president of Russia tells obvious lies to the media that's bad but not big news. When the president of the US does the same thing I really worry and it is a huge deal. (I'm not a US citizen but an ally.)

If I think of people in South Sudan doing female genital mutilation on their daughters, I'm a bit outraged. But if someone in my city did that, I'd be intensely and obviously furious - and more so if it was in my street - and even more so if it was in my apartment building, even if I never met the daughter.

If my wife were to make a stupid mistake that costs us a hundred bucks, I'd be angry. But if I (the closest possible person) made the exact same mistake, I'd be much more angry at myself.

I can't think of any counterexamples.

For each of these examples I could find more situation-specific explanations why I'd be more outraged by one than by the other. But it is more parsimonious to think the explanation is always in the degree of closeness to the perpetrator.

It seems to me like there's a one-dimensional rank order (or possibly even a metric) of closeness that co-determines the intensity of my outrage. It goes something like me -> my close family -> my close friends -> my extended family -> my acquaintances -> my fellow citizens of my city, my region and my nation -> fellow members of my culture -> fellow humans -> fellow mammals.

So an increase in felt outrage is exactly what you'd expect to happen if people were treating each other as closer than before.

Here's why this is an optimistic explanation. I think this rank order of closeness is closely correlated with, or even identical with, ingroupishness. This turns the binary ingroup/outgroup distinction, which is somewhat well-studied, into a more speculative rank order (or perhaps even metric) of ingroupiness. I think that's a pretty solid assumption and I vaguely remember ideas like this being discussed in the rationality cluster before.

It makes theoretical sense to expect outrage to be an emotion that's more likely to happen in an ingroup. If the perpetrator is outgroup, you can react with enmity: "if I ever meet you I'm going to kill you". If the perpetrator is ingroup, you can't do that (as readily) because that'd turn you into an unreliable ingroup member. However, you still need to signal that you don't agree with the act, so you don't share the blame as a member of the same group. Outrage does that. It provokes moral conflict that may help build moral consensus that you'd never bother to build with someone who's outgroup anyway.

So what I'm saying is the outrage epidemic means we're treating each other more ingroupishly rather than outgroupishly. Female genital mutilation is now done by people to people, when 200 years ago people in my country would have seen that as the barbarous customs of mere savages. The president of the United States feels like an acquaintance. A random stranger who did something terrible enough to make the news feels kind of like someone living in my town - I never talked to that person but I know their face and I can hear people talk about them. And the same is true of their victims. Even wild animals have now got a few Effective Altruists wondering about how to reduce their suffering. It is like ingroupishness is expanding in general.

And that's great news because as we all know, we treat ingroup(ish) people far better than outgroup(ish) people. We don't go to war with the ingroup, so the way to world peace is probably something like the way to treating all humans more ingroupishly. As we integrate with each other ever more closely and stumble towards some type of global hive mind, there's a lot of moral conflict to be had so we can build sufficient moral consensus in order to be comfortable in increasingly tight ingroups together. So that's going to involve even more outrage - but I think that's a price worth paying.

tl/dr: Outrage is felt more strongly towards ingroup than towards outgroup, so increasing outrage indicates we see other people as increasingly ingroup-ish.