にさせる usage in a sentence

Question

I was reading the Japanese version of No Longer Human by Osamu Dazai, and this sentence came up.

嫌な気持ちにさせるのだ

I would like to know why に particle was used before the word させる.

Answer 1

This grammar can be broken down into two parts:

First of all, にする is a grammar point on its own (see https://www.kanshudo.com/grammar/%E3%81%AB%E3%81%99%E3%82%8B for reference), which in this case carries the meaning of causing a state to be so.

The verb する is then changed to させる in its causative for, emphasizing the negative nature of the 嫌な気持. (See: Difference between にさせる vs にする（as in 幸せにさせる vs 幸せにする) and Difference between 人を不安にさせる and 人を不安にする)