Littering.

When someone carelessly throws their trash on the ground, that says a huge amount about their respect for other people, their feelings about the environment, and even their views on social equality.

It's a tiny thing, but an immediate dealbreaker.

People who throw their trash on the ground are the same people who yell and get mad at minimum-wage staff, while those staff hold back tears. They are the people who take more food at a buffet restaurant than they could ever even eat. They are the people who think the world and everyone in it owes them whatever they want, but without ever giving anything back.

I bet we all know a person whose car looks like a scary biohazard of old drive-through cups they haven't cleaned yet, but I'd much rather date that person than someone who throws it all out the window.