That is a very good point Rick. I think that there might be a scientific name to it but over time people kind of get used to something getting dirty or might not even see it happening but to someone that sees this stuff like we do, we spot it from far away.
It is kind of like when I first started pressure washing, I would start to notice dirty concrete all over the place just about everywhere I look. The same thing happened when I started cleaning roofs, everywhere I look I see dirty roofs.
Sometimes people just are not aware of how dirty their place is until it is pointed out to them, sometimes it is embarrasing to them where others are just appalled.