Because of the two years of war experience he's had throughout his life, in a year's interval each, with a three year gap between them. >_> I honestly think the ending was just there for the cliche, not any reason more. >_>Ender's Game was different; wasn't Ender unable to go back because his brother basically took over the Earth as well? And he wanted to help the Buggers rebuild, on top of letting the other thirds live in peace on their own planet. It's not like he completely vanished like Ike did, either.
This, also.For the stability thing, things were already pretty unstable to begin with. It's not like stability will return in a heartbeat especially considering the war that was occurring. Maybe it didn't return for a while and after doing stuff, he got bored and left for his retirement or something..?