The button group at the lower right corner is where we start looking for our button. Inside that button group, which is quickly located, buttons are parsed left to right.

To add to this, if we're doing a task, it's probably safe to assume we're more interested in going on with it, so there's a reason for the affirmative button being at the left of the group (first to be parsed). Also, you typically ask "Yes or no?", not "No or yes?". Just a maybe different perspective from a Windows user.