Why didn't Severus reveal the Prank? (Was: How did Sirius lure Severus...)

[potioncat]

No: HPFGUIDX 107149

Kneasy wrote:
> 
> Humiliation? Why would Snape feel  humiliated?
snip

>> And I  don't go along with the 'life debt' theory. 
snip

Potioncat:
I've cut and pasted from post #107020 (Another Chronology..)
because one of the questions in this post was: what made Snape keep 
quiet about the prank?, but that post has trust issues as well.
 
Nora is commenting on the scene in the hospital wing just after 
Black has been turned in

> THIS is where Snape starts to go behind DD's back to do something 
he knows his boss wouldn't approve of, and tries to get Black taken 
care of before DD can 'cause problems'.  This is the explanation for 
the quote in the hallway with Fudge--I don't have it in front of me, 
so  anyone who responds can plug it in, please.  The escape happens, 
> Snape is furious, and DD lets him fume because he knows that Snape 
> was up to something wrong; it's worked out later, given the end of 
> GoF and the reaction there.


Potioncat:
So we have Snape who has kept quiet about Lupin and the unfortunate 
incident all these years.  Even when he tells Harry about it, he 
doesn't reveal anything.

Now he's in Fudge's pocket. Eager for the execution, eager for the 
Order of Merlin. He learns Black has escaped and throws a fit. DD 
gives the word and Snape backs down and stomps off.  The next 
morning, after all these years, he lets it slip that Lupin is a 
werewolf.

Snape and DD aren't always in agreement in these books.  Several of 
us have thought of Snape as DD's right hand man, but I'm not so sure 
now.  He's starting to look a lot like Percy.

And in GoF, is he staring at his reflection in the Foe-Glass because 
he isn't sure which side he's on?

So, how has DD kept/earned/gotten Snape's loyalty?  And how much 
longer will he have it?