The Gleam! The Gleam!

[pippin_999]

No: HPFGUIDX 139113

Dumbledore knew, when he heard that Voldemort had taken
Harry's blood, that  Voldemort would excuse Snape for not
offing Harry when he had the chance, and therefore Snape could
indeed resume his role as spy. Thus the gleam of triumph,
and thus the reason said gleam has not yet been explained.

If we knew why Dumbledore regarded Snape's re-insertion
as his ace in the hole, we'd have a reason why Dumbledore
would be willing to die to keep Snape's cover intact, so 
obviously we can't be told.

Pippin