Like Manz said...
The stuff within alcohol and weed
lowers your inhibitions.
Definitions of inhibition on the Web:
- (psychology) the conscious exclusion of unacceptable thoughts or desires.
Then when your inhibitions are lowered... you say, think and do things you normally wouldn't do.
Like how drunks are either really emotional or do retarded things... like sleep with people they wouldn't ("beer goggles"), or start fights over petty things.
Your inhibitions are what keep you from acting upon impulse, and what make you think before you act.