Cas tried dope for the first time when he found a stash on a dead teenage girl. It let him forget everything for a bit and he made a pass at Dean, but they were traveling with a group of other survivors and Dean pushed him away, embarrassed by his intoxicated ward.
They were the only two in their group to survive the next day, and Cas laughed at how futile Dean's embarrasment was.
No one left to remember now!
He was truly falling off the deep end and it was fun.