Always assume that there is more going on in someone's life, interior or exterior, than you know.
No, really, always.
The people you live with count for this. The people you live with whose age is in the single-digits or measured in months: they still count for this. Because they will pop out with something you had no idea they had been working on or mulling over until all of a sudden there it is. This is why small children can still startle us with their questions; it's why we can still talk to our partners after years together. We never know every last thing going on with another person. We couldn't, and we shouldn't. Anyone who has someone who knows everything they're thinking about needs to do some more thinking.
This is not exclusively negative. I don't mean that everyone has things bothering them in the inner recesses of their own head, necessarily. I just mean that it's a good idea to keep in mind that we don't always know everything we think we know.