What made him think that it was a happy news-? Like were you expecting him to dance with j...

well, since his mind not working as a usual human(let's not forget that ml is freaking tiger), he expected that news will bring the joy since uke's curiosity would be confirmed and that he'll finally know what with brother. it's somehow psychopathic but yes, that's what we have for now and wait for his humanity to slowly rise with love ¯_(ツ)_/¯