he wants seoyoon to end up with a woman whom he maybe "set up" for him, so even if seoyoon got a family on his own, yeonjae still has the upper hand and got a grasp or control in seoyoon's life.
i bet his unhealthy obsession might lead to seoyoon cutting him out of his life lol jk (i wish)
oh that explains why siyeon is asleep most of the time in the chapters. i thought it was weird and thought the artist didn't arrange any lines for siyeon hahaha
eren what the fuck are you doing here