Saying "bless you" after someone else sneezes is superstition, not manners. It's a remnant from a time when people thought someone was losing their soul when they sneeze.
@xianc78 i've never heard that theory before. i've always heard that it popped up because of the plague so people would say "bless you" to give them a blessing of health.
if someone sneezes today, the same thing applies. you are blessing them for their health.