This is the viewpoint that confuses me, depending on what you mean by “default” behaviour. Do you think the everyday altruism displayed by people are exceptions? Or do you believe those people aren’t actually being nice to you/others and have ulterior motives? Such a viewpoint must make the world look like a very sad place indeed.

You may argue that altruism is actually in self-interest, by making you feel good, but that’s circular logic[1].

[1] en.wikipedia.org/wiki/Psychological_egoism#Circularity