The meaning of "Should"

When someone argues that you "really should" do something what are they saying?

One way to interpret should arguments is to consider them incomplete -

they may be seen as implicitly asserting conditional propositions: being of the form

"I think you think X is desirable, that being the case - facts A,B,C imply that you would logically intend to implement C, D and E"

Or "I think X is desirable, therefore facts A,B,C imply that you would logically intend to implement strategies C,D and E"*

Or more cynically one can interpret it as a direct attempt to control.

Is it legitimate to use the speakers degree of knowledge of

1) the person desire for X
2) whether X is already true
3) situational facts about that person (A,B and C)

How legitimate is it to use that to infer what they mean by "should"?
Or should we charitably accept that the first definition holds?

* where C+D+E given A,B and C cause X