I think the reason for it is grounded in theories of causation in human emotions/behaviors: there are a number of behaviors which are deemed to behave like 'sym

I think the reason for it is grounded in theories of causation in human emotions/behaviors: there are a number of behaviors which are deemed to behave like 'symptoms' in that they are effectively determined by a smaller set of underlying emotions, e.g. fear. Part of the idea, from what I can tell, is that the emotion (e.g. fear) in many cases doesn't appear at the surface level because it is "unacceptable" to a person, so their mind/brain assists in disguising it. But—the symptoms it causes are still there, hence this process of associating (visible) symptom sets to (non-visible) root causes, so that a non-superficial way of dealing with the problem might be recommended.