Harry's assumption VS Everyone's assumption

[Arief Hamid]

No: HPFGUIDX 151543

Hey all,

I just finished re-read the first 5 books in order to
have a better insight before reading the sixth book. 
One thing that's confusing me most, how come Dumbledore
trusted Snape so much and seems like ignoring the
statements and facts that given by Harry, that Snape
is nothing more than a treacherous member of the
order. What makes DD believes that Snape's is worthed
every bit to be in the order and will protect the
order with all his might. 

Damn, I couldn't find the reason why in the books, and
it just kicks me in the nuts.

Arief Hamid