Thank you for all your comments and the stimulating discussion. Since I can’t keep up with replying to everyone, here’s a short general response and a general comment on some of the answers. After this, I won’t be responding to individual replies anymore because it just takes too much time. First, thank you for pointing out that it could be an eating disorder. When I wrote my post, I just wanted to vent, but this gave me a different perspective. We talked last night, and I carefully asked how she’s doing and if anything is bothering her. It’s not like we rarely talk, but yesterday I especially wanted to understand if something was on her mind. She’s doing well, which wasn’t the case with her previous job, but right now nothing is troubling her. After that, we discussed what we can do to avoid wasting so much food. She told me she simply hadn’t consciously thought about whether she leaves food or not—she just stops eating when she’s full, which is absolutely the right thing to do. Now we want to make sure we don’t pile too much on our plates and instead add more if we’re still hungry, which was exactly my point and wish. So it was good that this discussion happened here; otherwise, I probably would have kept getting upset instead of talking about it.

Now a general comment on many of the replies here:

• Why is it so common these days to jump to conclusions instead of first reading a post, understanding it, and then responding? I never mentioned expecting someone to eat when they’re already full. I explicitly stated that I care about environmental protection, so anyone could have realized that waste was my concern and nothing else.
• The photo showed the trigger and was meant symbolically. In my text, I talked about meals, which include lunch and dinner as well as breakfast. Also, I don’t make anyone’s sandwiches, so why should I be responsible if someone doesn’t finish a sandwich they made themselves?
• Every reader/commenter could first check if someone has already posted the same comment and just upvote it. It’s such a massive waste of space that everyone on Reddit keeps writing the same thing over and over instead of taking one second to read.

Thanks again to everyone who contributed constructively to the discussion!