There are many ways in which the mind represents the world as a whole and the individual objects in it. I could say that mental states have been determined by the minds representations of all internal and external data.
At any time humans are in some various mental state - i.e. happiness, curiosity, anger, excitement, dread, or passiveness. I guess a question could be why is it hard to have a mix of some mental states - such as a combination of happiness and anger? Some combinations of mental states are common and make sense, while others (like being happy and sad at the same time) are hard on a person or difficult to acquire.
That is why concepts and consciousness has a scope and a limit - because there are only so many combinations of mental states, and it is hard to achieve many of these states.