I consider myself to be the kind of person who can quite easily imagine myself in someone else's place. I don't know if I'm actually any better at it than the average person, but judging by the comment sections on social media and the conversations I've had with other people, I really struggle to get angry at strangers like many others do, even for things that anger is an appropriate reaction to.
This doesn't necessarily mean that I don't condemn their behavior, but that it doesn't provoke a particularly negative emotional reaction from me. I observe the world from a distance, and when I see someone acting differently, I generally can come up with a charitable story about why they act that way. While it doesn't usually justify the behavior, it at least helps me imagine why they're like that and reminds me that if I were in their shoes, I'd likely do the same thing.
This applies to cheating, violence, racism... Name a bad behavior, and I can come up with a story about what a person might be telling themselves to justify it. However, littering is something I simply cannot comprehend. I cannot wrap my mind around what a person is thinking when they're throwing trash on the ground for someone else to pick up. If it's something "minor" like a cigarette butt, then okay, I can somewhat understand, but tossing your McDonald's takeout bag onto the side of the road is completely psychopathic behavior to me. I don't think even the worst people in the world think of themselves as "bad" because they rationalize their behavior somehow. But if you throw trash into nature, you must know you're being a massive jerk.
Tl;dr: I want to hear the best justification for littering.
I'm pretty much the same way. You gotta remember though that anger is not a negative emotion. An irrational amount of an emotion is a negative emotion.
An emotion is just a driving force of your behavior. "Angry" is mostly there for us to take action about unacceptable situations. Thus getting angry at the weather doesn't make sense, since you can't change it.
However, someone littering is in a limited way under your control. Like you said in another comment, you can confront them and use your power for them to pick up after themselves. Saying you're not getting angry/irritated at all is the equivalent of not doing anything about it to most people. And I do think you're at least getting irritated (which is a low form of anger) at people littering, which is why you do something about it or post this topic.
It seems to me a bit that you see any kind of anger, including being irritated, as bad, and thus show no sign of it/deny it even though you have it.
It's obviously a negative feeling in my view. It can be used as motivation for changing things to the better but I can't see it as anything else as negative. It's not a pleasant feeling.
Ofcourse I'm not immune to it myself either but being such a strong emotion it's nearly impossible to not notice and thus it acts as a kind of mindfulness alarm. When I catch myself getting angry at something I immediately realize how that is in conflict with how I see the world and then the anger basically dissapears. It's kind of like waking up at the morning and being irritated that it's raining outside but then at the same moment realizing that I can't change the weather and I'll rather just be wet than wet and angry. It's the so called second arrow.
Unlike weather I can affect other people however, and I do. No disagreement there. I simply just don't see the need to feel anger while doing so. It's done and can't be undone. They couldn't have done otherwise so no need to get angry and pretend as if they could have. My intention is to encourage them to not act that way in the future.
I guess it depends on how much trouble you have with too much anger. Very often, anger is counterproductive, not only for yourself, but for actually effecting the change in others you desire.
In that case, it's often good to "overcorrect" and rather try to feel less anger than appropriate, so that your troubles get reduced.
However, I still think it's an overreaction and an appropriate amount of anger at the appropriate things is healthy, as in the end it's actually a conflict within yourself to have less anger than appropriate - you're forcing yourself to spend more energy suppressing/letting go of the anger, and then having less energy to effect the change you desire.