Because he thinks Yoontae doesn't know
Yoontae also joked that once he discovered his first love they would start a family right away. Jeongseo didn't realize that Yoontae was kidding about breeding, so he's keeping his mouth shut as they are too young to start a family.
Omg i need to reread that what chapter is it????
25 -- the breeding comment
36 -- Yoontae admits to himself it was a joke.
I only read the chapters by Kang Byeol so the officials might have worded it differently.
Didn't he already guess that he's yoontae's first love? Why's he hiding again