"A sensitive mother who gives birth to a robust, energetic, less sensitive baby might feel out of tune with him and think of him as aggressive; or, alternitevly, she might be relieved that he is so easy to settle and to take anywhere."
I think the author means that she might be relieved that the baby is easy to keep quiet/calm down (imagine how nice it would be to take this baby on a bus, or the movies!)
the opposite would be a restless baby (one that cries a lot and won't sit still).
