Sam was betting $20 that Dean and Cas would make out by the end of the week. Bobby was betting that it would happen earlier. So while normally Sam would be pushing his brother and angel together, this time he was hoping to push it out until Saturday. Just to win the bet and prove a point. When he walked in on Thursday to Dean and Cas half naked on the motel bed, he shouted "Dammit!" threw his bag on the ground and left to call Bobby.