*her
My guess is that he loves her because she fawns over him and overtly expresses herself towards his gifts and affections. She's childish in ways nobles can't ever behave.
Since the empress is 'cold' to him, Trashta's reactions are soothing to his male ego.
I think it might remind him of how the empress acted as a child and how she used to express herself more since he seems happy when ever we see flashbacks to their childhood and how happy the empress is then if you compare it to Trashta personality she acts childish and is emotional just like a child.
Yup, exactly this.
ヾ(☆▽☆)
Come to think of it that's maybe the true reason behind his over-affection toward her
Even I know that concubine's children can't be princes
Sorry but really why does he love her, I mean she's not that beautiful imagine when she was a slave and she's absolutely dumb so why the hell r u in love with you insane of an emperor