I think at the first part the emperor really did fall in love with him at first sight but because of how the Jin people doesn't know human's feeling and to treat them as a lover instead of one of his hobbies his action came out as someone who just playing around with his toys . Now that yeodom came back to the past and teach him (as a child )how human treat each other and feelings , he have change to someone who's gentle and cherish his lover .
That is how I describe the future emperor's behaviour . ╮( ̄▽ ̄)╭
I agree, remember the part of this novel where the future was narrated? The part where they published a book of some sort to clear out the terms used by jins to express themselves and it's human term equivalent? To jins they say that the ones they are into are their plaything whereas what they really imply is that person is their lover who have a special place in their heart.
I thought that sunbea is a teacher at first lmao