Caydon becomes very sticky to us

For the past 3 days when we hand him over to the teacher at ifc, he cried :(
I wonder why did he become like that? He has always been smiling happily to the teacher and don't really care about us leaving. But strangely as he grow older, now that he's 9mth+, he start to become very sticky to us. Even when at home, we also cannot leave him alone, he will cry murder!