Well, pretty much, Mui has already given the reason as to why he likes her so much when he went back to pick Soah up from the Human World. When Soah came back (and she was starting to remember the old memories), Soah even wondered the same thing. So just go back and read.
Does love ever need a reason? <3 (Yes, I don't get it either T_T)
________________ "I love you like the plant that never blooms yet carries in itself the light of hidden flowers. I love you as certain dark things are to be loved, in secret, between the shadow and the soul" - Sonnet XVII