If you're interested in learning the traditional use of "should" (and related words), you need to read the relevant chapter of Fowler's
The King's English (1908), namely "
Shall and Will". "Should" and "would" are the conditional forms of "shall" and "will", respectively. The sentence "I should adore this comedian" is essentially an abbreviated version of "I should adore this comedian if something were true which is not true, such as that I were similar to other persons".
Miles isn't exactly wrong about "should" being related to duty, because it's a conjugation of "shall". However, "should" is the
conditional form and so "I should do X" doesn't mean that I have a duty to do X; it means that the duty for me to do X exists if and only if some unstated condition is satisfied. The condition can be explicitly stated, but in this topic we can infer that the condition is basically "if I were similar to other persons".
As Fowler puts it, "[w]e do not ordinarily issue commands to ourselves", although if we wanted to, the traditionally correct way is "I shall", not "I should". The difference between "I will" and "I shall" is that "I will" is a statement of my current will (it means "it is my will"); whereas "I shall" means I am ordering myself to do something that is not otherwise willed by me. "I should" is different from either and expresses a conditionally contemplated duty, possibly without the condition being explicitly stated.
When Gandalf said "you shall not pass", he was not making a prediction about whether the Balrog would be able to pass; rather, he was
ordering the Balrog not to attempt to pass. If Gandalf had said "you
should not pass", he would have been issuing a conditional command to the Balrog, essentially saying "if you know what's good for you, do not attempt to pass". If Gandalf had said that, "you
will not pass", he would be expressing a wish that the Balrog not pass, or alternatively, he would be making a prediction that the Balrog would in fact not pass.
Approximately no one follows these traditional rules anymore (not even me), but it's still useful to understand them. In present-day English, these words have kind of all blurred together and the speaker's intent just has to be inferred from the context.