"when someone's this happy..." nah, that's non-canon, Yugami won't feel good from this. I know this too well
I actually had trouble translating that particular sentence since it felt kind of non-yugami-ish, so that version kind of felt the most okay.. But i might be wrong.
In Japanese it's "Anna ni yorokobareru to ureshii mon da na... あんなに喜ばれると嬉しいもんだな…" -> When someone's this happy, I guess it does feel good
I was also thinking maybe ""I didn't realize having someone be that happy would feel this good."
As it's something that was new to him as well.
Though if to keep the Yugami-ness maybe "It's surprising to see her this happy" would have worked better
If you happen to have a Japanese friend feel free to consult.. >_<