Juugo sees that Karin is interested and happy, and it makes him happier too. It's good that someone Nagato likes, likes him. It's nice to see people smiling around him too. And he likes talking about his friends.

"Yes. They've been my friends since I was small. My older friends were from before Ame, and they don't like the wet as much as my new friends here."

The birds didn't care quite as much about Nagato as Juugo did, and while they could feel chakra, it didn't mean to them, so they complained. But they liked Juugo, so they stayed, and it made Juugo very glad.