I always taught my children that. When they would see or want something, even from one another, I would ask them if it was theirs. When they said no, I would say, then it belongs to someone else.
I would never take something which belonged to someone else, and find it difficult to understand why anyone would think that it's okay.
Although, aside from people who might not have been taught any better, I do suspect that it could have to do with drug use. Methamphetamine users, for example, are well known to take things to trade or sell for more meth. So, that could be it. Maybe not, but that's what came to me today while I was contemplating it.
I'm sorry that your sinuses are bothering you.
Love All Life; Thank You For Posting! :)