Because so many people are wildly different, human interaction is extremely nuanced, and if you tried creating a rigid values system for all the people you come across, you'd drive yourself mad. To an extent, we all make concessions for people we like. Everyone is a bit biased at times. It's good to be aware of it, especially when it comes to making a decision that could have widespread consequences (e.g. hiring incompetent people because they're your friends), but I wouldn't agonize over it such that you attempt to create a rigid values system for every casual acquaintance.

As for why, it could be several reasons. It could be deliberate manipulation. The classic example would be the person who laughs at stupid jokes because they want to have sex with the person making the joke, not because they think the joke is funny. It could be unintentional cognitive bias. Maybe you feel more of a connection with one person because you have a shared interest or you're in the same group or whatever, so you allow more stupid jokes from that person than you would otherwise. Usually, I think, it's just because you know the person better. I enjoy very dark jokes, but I wouldn't if it was coming from a stranger (unless it's a real zinger ), for obvious reasons.

On the other hand, I don't find anything wrong with this sort of behavior:


... but apparently:


So probably I'm a bad person. I still think I'm right to an extent, and what's "appropriate" lies somewhere between me and bpcyclist. I'd be interested to see what others think.