Making life easier for the end-users. Sometimes, spoiling them with little nuances that make things easier.
No, seriously. At the end of the day, if the lives of the end-users that are using (afflicted with?) the software/systems are better in some small way, then I know it's a job well done. I know there are similar answers, but hear me out - mine's slightly different.
I think it has something to do with pursuing a personal work philosophy that is akin to Eudaimonia; and in Systems Administration, happiness in your work means pursuing excellence that makes your effort worthwhile, IMHO. The measure of that excellence will be in the satisfaction of the people that appreciate or use your efforts (given the nature of the job).
So, making systems better and better for our users = happier users = better lives for them and me.
And if everyone's lives are better, then I know as I head home, deep in my heart, that it was a good day to be a SysAdmin.