Why Simcha Bunim?

Question:

My grandfather's name was Simcha Bunim. I know Simcha means happiness, but what does Bunim mean, and why are these names a pair?

Response:

Some explain that Bunim is a Yiddishized version of Bonhomme, the French name meaning "good man" (similar to the German/Yiddish name Guttman). Accordingly, they write, there is no apparent reason why this name is paired up with Simcha.

However, I found a fascinating explanation brought in the Beis Shmuel.

(Authored by Rabbi Shmuel ben Uri Shraga Faivesh, a seventeenth century Torah scholar who served various communities in Poland and Germany, this work is one of the primary commentaries to the part of the Shulchan Aruch dealing with the laws of Marriage and Divorce. Since the names in a divorce documents must all be spelled accurately, as a supplement to the laws of divorce, the work includes a list of the correct ways to spell many common and not-so-common Jewish names.)

He writes that the name Bunim is actually a diminutive of Benjamin, Jacob's youngest son. After he was born, his mother Rachel named him Ben Oni, meaning "son of my pain," since she was about to die from childbirth. His father changed it to Benjamin (alternatively translated as "son of my old age" or "son of the right"), and this became his name. It is therefore common to join Bunim to Simcha, which means joy and happiness, to temper the sadness and pain with which it is normally associated.