Going hand in hand with the cultural norm of warmth, politeness, and well-wishing that brings you greetings such as "Have you eaten?" the phrase "Many happy returns..." accomplishes the task of acknowledging the special occasion while blessing you with many more to come.
Fell swoop sweetness is what it is. Welcome to our subcontinent.
(ED: We wish India (Aug. 15) and Pakistan (Aug. 14) Many Happy Returns of the Day!)
Sumant: "Cool cool. No problem. Bugger! Many Happy Returns of the Day!"
Jagan: "Umm... dude... I have 3 months to live..."
Sumant: "Oh...... Then? How are things otherwise?"