It's expected that you write thank you notes
Jack got married and his boss got him a very expensive gift.
Jack wants to thank his boss.
Jack write a handwritten letter to his boss thanking him for the gift.
Jack says thanks to his boss while passing him in the hallway at work.
Jack's boss will think Jack appreciates the gift and is grateful.
Jack's boss will think Jack is too casual and not appreciative enough of the gift.