Believing that it is simply common courtesy rooted in social etiquette, you may have given little thought to why people say this. Yet, the expression is rooted in superstition.
आप शायद यह मानते हों कि यह बस शिष्टाचार है या चार लोगों के साथ उठने - बैठने के तौर - तरीके हैं ।