The reason why you left because you had a terminal illness that can take their life anytime and you don't want him to be overly attached to you because you know that Kaito is capable of taking his own life if you ever die. so you used the excuse that you moved to another country and didn't said goodbye to him so he'll resent you rather than love you. so much resentment that he'll end up forgetting you but instead, Kaito's love for you didn't even got less in those months when your away