Why people don't clean up after themselves

This is one of my pet peeves. People can be so entitled, can be so unaware of what's going on around them and how other people manage to take care of their own business, plus that of slobs, that they never even realize it. Earlier today, there was birthday cake for a coworker, and the leftovers were put in the break room. Just now, walking through the break room, I noticed that the cake was all gone, but the platters were left out, full of crumbs and icing-covered plastic knives. Is it too much to ask that the person who takes the last piece of cake throw away the paper platter it was setting on? I asked this, rhetorically, of another coworker who was getting some coffee, and he sighed, and pointed to the half-full single serving container of creamer someone left on the counter. Are they trying to conserve resources in the hope that someone else who only wants half a cream will take it? Cause that will never happen. Or, more likely, they're just too much of a slob to bother to throw it away.

My SO said to me last night, "Hey, tomorrow's trash day, you better take the trash out." I gave him a dirty look, and he laughed, cause he knew I knew he was just playing asshole. It got me to thinking though, which would be worse, an SO who tells you it's trash day, and you better take the trash out, or an SO who has no idea when trash day is?