I have no idea wtf yasaka said. Stuff about selfish or whatever can someone explain in boomer terms
Dunno if this helps, but basically the girls were all trying to read in to Shima's actions to figure out what his angle was in asking Mitsumi out.
Yasaka suggests maybe there was no 'angle' or he didn't care about the implications, he just did it cuz he wanted to.
Mitsumi says isn't that insincere (I think you could also say shallow/flippant maybe), since it's an action made without any thoughts behind it?
Yasaka says no not everyone thinks things thoroughly like that, in fact, because he just did what he wanted without thinking, that probably WAS his genuine true feelings.