Why do people act so stupid?

i think some people act crazy just to get attention from some one.or they might do it because that person was in a same situation like that.

well, i act like an idiot around my friends and they think im hilarious,and when i get called weird or stupid, i say, thank you!

we are all sometimes act stupid especially when we are in good condition to think of

its obvious they have under developed brains like a cave man but on the bright side they might survive the zombie apocalypse

No one does anything because they think it's a bad idea or that it's a stupid thing to do. They always have some underlying reason or belief that they should, even if it's without much thought.
So, to them, they aren't acting stupid, just being themselves. That means they are either stupid to start with or that they acted impulsively, the latter of which draws more questions about themselves.