In an episode of Downton Abbey,one of Lord Grantham's daughters says"I'll leave home and then you'll be sorry" He replies"I should be sorry" He doesn't mean he ought to be sorry,he means he would be! When should and when would?
Minefield!
I/We should
You/he/she/they would.
Can't remember the particular episode but it depends on the tone of voice as to what he meant. 'I should worry' means he would worry. 'I (stressing the I) should worry!' means the opposite.
"I" (1st person singular pronoun) has been moved within the sentence to remove the "?" from the end.
So it could originate from, "Should I be sorry ?" said rhetorically.
It has a ring of a Jewish/Yiddish origin, said with the shrug of the shoulders.
I've never seen the programme though.
The trouble with 'should' is that it has two meanings, what might happen or what ought to happen. In the latter case 'should' is used for all persons, as in, 'He should have gone to Specsavers'.