Thanks to everyone who has taken the time to respond. This has been both interesting and helpful. While pondering your input I've considered my own mental mechanisms and thought about why just loving to ride hasn't been enough for me, and I've come to some realizations that weren't clear to me.
1. Driving -- This is the one where I was most surprised to realize what's going on a lot of the time. I really do enjoy biking to work, and depending on traffic my attitude toward driving ranges from dislike to detest. So why would I ever choose to drive when I don't have to? It's easier. Not more fun. Certainly not less stressful. Just easier. Despite my involvement in cycling, I am a profoundly lazy man -- quite possibly the laziest in Washington County. As many of you have said, I never regret biking to work once I've done it, but there is that time before I've decided to do it when some days I just don't feel like doing anything -- and let's face it sitting in a car driving is pretty darn close to doing nothing. I think here the advice from those who said not to think of it as a choice is the answer. If I don't ask myself whether or not I feel like riding to work the answer won't be 'no.' What's more, I know from experience that the more often I ride the more energy I have so this one should diminish with time if I can get myself re-established.
2. Sleep -- I really enjoy sleeping, or perhaps I should say I enjoy rolling over and going back to sleep after I wake up. This has always been an issue for me. My senior year in high school I got an automatic 'F' one grading period in a class that I was crushing academically because it was the first class of the day and I missed it more than the allowed number of times. When my daughters were in school I had something external to regulate my schedule. Right now I'm on my own and not adjusting well. Luckily my job allows me to be a slacker in this way. Today I just decided to bike in even though I felt like I was "late" and nobody said anything. That doesn't feel like a good solution. Just "deciding" to get up early and get moving doesn't seem likely either.
3. Schedule conflicts -- Thanks to Darth Lefty for mentioning juggling kids. A lot of my "skips" over the past few years have been because of family issues. Sometimes it has been having to take my daughter to a late morning doctor's appointment. Sometimes it's a family event going on early in the evening so I want to get home as early as possible and not be all sweaty when I get there. I'm not sure what I can do about this, but it wasn't an issue in 2009. Maybe getting to work earlier would fix it.
4. Weather -- This hasn't really been brought up much, but it's been a factor for me in the last six months. I always used to think of myself as a rain or shine commuter, but the weather really wore me down this winter. I've lived in the PNW for 22 years, so I know how to accept a lot of rainfall, but this year has been something more than that. It's probably more or less back to normal now, but I'm so shell-shocked that I find myself looking at the weather forecast and thinking "no more." I did mention maybe not riding home last Friday because of such a glance at the forecast. It ended up not raining at all during my ride. Last Thursday it rained for my entire ride in to work and the beginning of the ride home. I can get over this. I just need a bit of summer.
5. Fitness -- I know I initially related this to the size of my gut, but I really do see fitness and weight as two separate things. The main thing I hope to get out of riding more is a better physiological relationship with the bike. I want to restore my base fitness, regain the endurance I used to have (riding all five days last week wore me out!) and maybe pick up a little bit of speed. Weight loss is something else. I do find that when I'm riding regularly that makes losing weight extremely easy, which is nice, but it does still require some adjustment to my diet. I'm doing that right now, but losing weight really isn't my primary goal. My fitness dropped off so much in the last two years and I was having such trouble getting it back that I went in to talk to my doctor about it, thinking there may have been a heart issue behind my seeming inability to regain the form I once had. He told me that at 47 I couldn't expect to do the kinds of things I could do when I was 25. I told him I just want to be able to do the kinds of things I could do when I was 45. A few tests confirmed that I'm just out of shape. I can fix that.