I always heard that 420 made you mellow and crave munchies not act like your arse was on fire. . .
Then you only heard part of the story.
If you are basically lazy and love to eat you will
almost certainly do less and eat more; and
most people are basically lazy and like food. This is the typical situation comedy reputation of marijuana, and like many reputations it's only a half-truth.
Just like drinking alcohol, or taking many other drugs, it generally turns loose the part of humans which civilized people normally keep under wraps.
Like a baby version of sodium pentathol, what's really inside you will tend to come out; or as the old Romans used to say: "in vino veritas."
Even tempered persons will tend to be more even tempered, while a radical person will tend to be more radical.
If you like to talk to you will tend to blab. If you're basically shy you'll usually want to fold up alone like a Wallflower.
If you are basically mean, you can feel and act meaner. Things which have been boiling up inside of you can finally boil over. If it's the spirit of Rambo that's inside of you, you're going to tend to Rambo up.
Finally, like alcohol it is, generally speaking, very bad for your health and I wouldn't advise using it.