An outside observer can tell you what's going on outside your body, but if you want to know what's going on inside, you have to ask.
And it's true the other way too. You can't expect your friend to know how you feel unless you tell them. So many misunderstandings happen when people think they can read other people's feelings from the outside, and when people expect others to anticipate their needs and wants.
I'm sure it must seem like common sense, but it's amazing how many relationships are damaged by not accepting this limit -- people can't read minds.
These expectations probably come from infancy and parenting, when a baby has limited ability to inform the parent what its needs are and the parent has to be good at guessing. But a baby's needs are simple. Feed me, hold me, change my diaper. Adults are (we hope!) more complicated than that.