So if it's because "he's gonna be a caring father" - shouldn't the congratulations and back-slapping occur after a period of time, once that a father has proved himself worthy of carrying that title?
It's easy to say you'll look after a cute little baby when it's all wrapped up in a tiny bundle in the hospital - it's another matter when things get tough, when the kid is playing up, when the kid becomes a trouble-filled teenager.
By that logic, then we should'nt congratulate the mother, either. Lot's of them F up the baby while it's still in the womb, alcohol, drugs, etc.
DO you have kids? just curious