What is this "should?"
Should is a modal auxiliary verb, which is a very special category of auxiliary verb in English.
can, could, shall, should, may, might, will, would, must, and sometimes need and dare
Modals have a number of strange characteristics
One of these is that they are systematically ambiguous.
All English modal auxiliaries have at least two kinds of meaning:
a deductive or inductive logical possibility or necessity, called the Epistemic meaning.
a moral permission or necessity, originating from natural or social forces, called Deontic.
The epistemic sense of should is something like "likely by induction", not all that different from the "likely by deduction" of epistemic must. Both are used to announce hypotheses derived from theory or experience.
- It should be sunny tomorrow.
- He should be home by now.
- He must be home by now.
- This should be the store he mentioned.
- This must be the store he mentioned.
The deontic sense of should is something like "mild obligation", contrasting with the "strong obligation" of deontic must. It's usually a matter of reminder and nagging, rather than order and obligation.
- Men should never weary of goodness and humility.
- You really should get your gutters cleaned before the snow starts.
- I know I should do it, but I just never have time.
- You must have your gutters cleaned or pay a fine.
Reminder: All modals have two senses, not just should. They're easy to tell apart once you learn.
The first statement is more like a moral standpoint, in the sense what,in an ideal state, should or should not happen.Something like a commandment.
One should not steal.
The second statement is more like an educated guess:
It should be sunny tomorrow.
would read something like:
*Given the weather conditions today, I assume it would be sunny tomorrow.
(Given my performance), I should clear the interview.*
The main purpose of should is to express obligation. That is what it is doing in the first sentence. A secondary purpose is to express deduction. That is what it is doing in the second sentence.