You know that part of late Spring when it's quite warm outside, but the pool water is still quite chilly? If you have kids, you know there is no way you can explain to them why it's still too cold to go swimming.
"Blessed is the man who walks not in the counsel of the wicked, nor stands in the way of sinners, nor sits in the seat of scoffers..."
See the progression...walk, stand, sit? The idea is that no one "falls" into sin. We sin our way into deeper sin. The choice to ignore the opportunities God gives us each day in favor of opportunities that are not of Him puts us on a course towards the seat with scoffers...and scoffers are godless people who literally shake their fist at God and dare Him to get involved in "their" lives. You don't get there overnight...it's a progression. It starts small and then it grows until it outgrows you and it can't be hidden anymore.