Reason Dumbledore trusted Snape.

[justcarol67]

No: HPFGUIDX 171676

Eggplant wrote: 
<snip> I'm not convinced Snape turned to the good side, a slight tilt
in that direction over the years maybe. In my theory Dumbledore trusts
Snape because years ago Snape made an Unbreakable Vow to protect the
life of Harry Potter. Dumbledore did not know that Snape made this
noble sounding vow for less than noble reasons.
> 
> Suppose Snape heard every word of the entire prophesy but Dumbledore
sincerely believed he only heard half; we know for a fact he only told
> Voldemort half but I think the reason is that Snape knew attacking the
> baby Harry would be very dangerous and he wanted Voldemort dead.
> Suppose Snape wants to rule the world but 2 wizards stand in his way,
> Dumbledore and Voldemort. One down one to go, and that's why he needs
> Harry alive until he can finish off the Dark Lord.  

Carol responds:
I've said this before, but here I go again. Trust is not based on
compulsion. It's the belief that the other person will do the right
thing of his own volition. Note that DD did *not* trust Tom Riddle,
but he trusts Hagrid "with his life" and Snape "completely."
(Probably, he doesn't trust Hagrid with secrets--that would be utterly
foolish--unless he for some reason wants those secrets to "slip.")

Nor do I think that Dumbledore, who believes in Love as the most
powerful form of magic and in the importance of choices, would bind
*anyone* with an Unbreakable Vow, which not only involves compulsion
but, AFAWK, the death of the vow breaker. That sounds like Dark Magic
to me--the sort of thing that DE supporter Narcissa and DE Bellatrix
think nothing of, but DD would, IMO, never consider, especially if
McGonagall is right that there are kinds of magic he would never use.

I agree that Snape is protecting Harry because he knows that Harry is
the only one who can vanquish the Dark Lord. Perhaps that, in itself,
is the basis for DD's trust. He knows that, for some reason known to
him but not to us, Snape wants LV dead. And, IMO, it's most unlikely
that Snape wants to be a Dark Lord himself. While he does enjoy his
power over his students in the classroom and for some reason the DEs
cowered when they saw him on the tower (did they sense that his power
was greater than theirs? Did they fear his anger? Was it just that
they thought he was LV's "favorite"?) I see no evidence anywhere that
he's recruiting followers or trying to take LV's place. Surely he
would have done so long before when LV was powerless and the DEs were
looking for a new Dark Lord to rally around if that were the case. (I
see no evidence, either, that he sees the "mediocre" Harry as a rival
for anything except Dumbledore's trust and affection--sibling rivalry
with favorite son James transferred to new favorite son Harry.)

Carol, quite sure that Snape is loyal to DD and that DD's trust was
justified and hoping that the chapter near the end that made JKR cry
is Snape telling his story--and Harry believing him