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 wonder if it would help to think back to the first time you littered? When I was 5 or 6, I remember eating a candy and not wanting the wrapper any more. It had to be someone else who saw what I did and pointed out that it isn't good if we all did this, because then the playground would be all full of trash and we couldn't play there. I was like, "Oh, I get it." But if someone hadn't explained it to me, I think the behavior could have innocently continued for quite some time. I grew up in a very rural place (northern Canada).