That's a good question. In the earlier SG-1 episodes they were set up as saints and angels, kind and smart. They even with the power to heal through touch. Then they became stoics or skeptics by season 8, and by the time SGA came around they were jerks.
Wasn't it all tied into their arrogance being their downfall?
Otherwise if they remain as saints and angels it's hard to get a believable downfall going.