Jim Webb emails me (and everyone else on his list) to tell me he isn't running for re-election.
Obviously I don't fully know his reasons, but one thing I've learned is that being a senator probably isn't very much fun, at least for most people. To do your job minimally requires a lot of work. You oversee multiple offices of largely underpaid staffers, many of whom are doing the mostly pointless work of handling constituent correspondence to maintain the illusion that Your Senator Is Listening To You. It's the Senate, so if you care about getting things done you're inevitably going to be very frustrated. Especially if you live far away, there's a damn-if-you-damned-if-you-don't issue about where you live. If you move your family to DC you've moved out of the state you're supposed to represent. If you don't move your family to DC you have to travel hundreds or thousands of miles to see them. Your schedule is complicated and your life is basically run by your scheduler. It's a very high profile position, which means having a private life is almost impossible. At least House members can go about their business in relative anonymity if they want to. And of course there's all that fundraising.
To actually enjoy the job you have to get some enjoyment out of the semi-corrupt aspects of the job such as the fundraising, hanging out with lobbyists, the general thrill of everyone kissing your ass, etc.