Once upon a time in the early 1990s we discussed the limitations of our ADnD campaigns -we had dabbled in Warhammer,Top Secret,Bushido,Paranoia,CallofChutulu etc .I saw an ad for T2K 2nd ed in I believe it was Dragon Magazine.
It was promptly ordered ( I dont recall if we went dutch ,but I think I paid for it ) .
Our first session was a shocker- a PC got himself killed!!!Players were behaving badly !Machineguns were bursting left and right non stop.
The first few years of T2K-ing saw intense infighting with deadly scraps between player chars over small things - as you guys discuss-tempers grew short .PCs dropped like flies to eachothers bullets and buckshot.One time 4 out of 5 PCs wacked eachother when the mexican standoff they were in with MK19s and LAW 80s went wrong.
Those highschool years gaming was about getting an interesting PC with really good stats and the best guns possible.A session was considered a success if there was a huge gunfight.
Over the years other elements crept in -such as the need for a plot and ambience that was skillfully done .The"old us vs them over the treasure "grew old and oposing interests were added to the PCs backstories to make it interesting.The game became more orientated towards creating a rich gaming experience-gunfights and rewards didnt cut it anymore.
As we grew steadily more mature ( in the sense that grown men playing RPGs are

) a hankering for some more serious themes arised and the questions risen from the topic title -what could possibly motivate after WWIII?
Our group found our answer and wrestled with that for the last 5 years of our FtF - power.
All of them set out to build the brave new world from the ashes of the old one.Police states,religious fanatics,feudal oppression,slavery and racism became the result .As well as a fair bit of inter family crime like spouse- murders,father/son civil wars,incarcerations,violence ,threaths etc .
I would like to think the group has come
almost full circle in regards to motivation.Our gaming has certainly done so-from just a few hastily scribbled maps and some dice to bigger set ups with computers,laserpointers ,maps that have taken ages to draw,cast miniatures etc .From where its all about your PC and what he carries in his backpack to where the PCs run entire factions with XL sheets and micromanaging.
Next FtF in 20 days.Cant wait to see where it takes us in game.
I think the topic has many answers - some may act as they do out of habit ,not wanting to realize that the old world is gone,some out of self interest some out of altruism.The list goes on.
Good thread !