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 think it is the convenience and there is no punishment for littering nor the reward for not throwing. Yes there is environment but it's like a collective thing. It's something else when a person would instantly connect that it's his/her action did that. There is also how they learned it from their peers or parents. Doesn't help when it's tolerated in the society. Personally makes my blood boil seeing someone littering, the most I would do is just pick it up myself while the person is there. Rather than making a scene and ask him/her to pick it up. That way I give shame to his/her behavior.