because when someone's silent and possibly upset, being nice to them lets them know you are still on their general side too even if you specifically disagree with them on something. Lot of people aren't good with managing their emotions, and are socially clumsy. To be frank, people need help socially sometimes. Waiting for everyone to reach a bar whose height is meant for everyone is what doesn't make sense (e.g. waiting for them to apologize or admit they were wrong, things like this. For some people this can take months, some people will never admit it which will create permanent distance in a friendship that's annoying if the two people can't forget about it), you meet people somewhat where they are. Sometimes this means being nice to someone who is mad perhaps for no reason at all or is mad for an undeserving reason. I'm not hear to argue about the merits of either approach by the way, I'm just answering the question in one way.