Out of Habit
Being polite is something that you learn at a young age, and many manners are second nature by the time you reach adulthood. For example, saying “bless you” when someone sneezes is considered to be good etiquette.
If a stranger sneezes when you’re out in public, it may not be appropriate to offer a “bless you.” This person kept it quiet, but her boyfriend still picked up on the habit and didn’t find it very welcoming.