Rejoice is a bigger word than happy and it is doing something different. Happy depends on how the day went. Rejoice is a decision you make about what is true regardless of how the day went, which is why it turns up so often in verses written by people who were having a genuinely rough time of it.
The original scene was not grand either. It was a small, cold, deeply unglamorous night that people have been singing about ever since. O come, o come is a phrase of waiting, and Advent is essentially a month of practicing it. Most of the year we are bad at waiting. December gives everyone a structured excuse to do it on purpose, with candles and a countdown and the understanding that the good thing is coming whether or not you can see it yet.
That is a useful rhythm to have in your hands during a season that otherwise moves very fast.
A word worth carrying in the weeks when it is still dark early.
