a thorn in one's flesh
A: The stepmother was very couldn't stand a happy smile on the boy's face.
A：那个继母坏透了，她看那孩子笑一笑都难受。B: And?
B：那怎么样?
A: And she did her best to pull out the thorn in her flesh and the boy lost whatever made him happy.
A：她想方设法除去她的眼中钉，那孩子生活中的快乐全没有了。B: Poor child!
B：可怜的孩子!