「彼は瞳をキラキラさせながら言った。」 Why させながら, and not しながら?


> 彼は瞳をキラキラ **させ** ながら言った。

I think that it is "He said with shining eyes", but what's a word "させ"? Why
"させ+ながら", and not "しながら"?




キラキラする is intransitive "shine". eg 「瞳がキラキラする」"his eyes shine"

You use causative キラキラ **させる** "make something shine" here. eg
「瞳をキラキラさせる」"make his eyes shine"

> 彼は瞳をキラキラさせながら言った。

You can split this into two actions:  
「彼は瞳をキラキラさせた。」"He made his eyes shine" +「彼は言った。」"He said."

So the sentence literally means: "He said, while making his eyes shine." Hence
the translation "He said with shining eyes".

* * *

When combining two actions with ながら, you should use the same subject for both
verbs. i.e. you can't use two different subjects for the two actions, eg
「彼は、瞳がキラキラしながら言った。」 is incorrect.

