Rule of Perception

In a lot of contexts, what's actually going on is considered to be more important than any particular individual's perception. However, in terms of the psychological impact on the individual, perception matters much more than reality.
So, for example, let's say we have two children. Child A's parents don't love him/her, and show this clearly. Child B's parents love him/her very much, but don't show this clearly. Both children are going to feel unloved and show the same set of psychological problems (all other things being equal) because they both perceive their families the same way. So, the fact that Child B's parents love him/her doesn't really matter that much to to the outcome*, what matters is how well they show their love.
For another example - imagine if a person is paranoid. He/she will be under the same sort of stress as someone who is really being plotted against, even though it's all in his/her head.
So if you want to understand why someone is acting a certain way, and what's going on in that person's head, knowing what situation they're in won't necessarily help. You need to know what situation they think they're in.

* It does matter in one way - parents who love their child but aren't showing it very well are more likely to improve over time. If a psychologist tells both sets of parents that their children feel unloved, Child B's parents are much more likely to see this as a problem, and it's a whole lot easier to choose to start showing your feelings better than it is to choose to start feeling something new.