The focus in usability is often on guidelines, dos and don'ts etc., the reason being, that if these guidelines are broken, the user usually has difficulties using the system. And then the explanation stops there. It is rarely discussed, why this makes it hard for the user. Why are some things just so darn hard for the user? What exactly is it, that makes it so hard? This talk will go into the psychological theories and explanations, that lie behind the difficulties users experience when guidelines are broken. By understanding these psychological explanations, we in turn become better at understanding what to strive for and what to avoid, when designing interfaces, because we understand why.