Don't beat yourself up over it. My goal is to bike commute 4-5 days/week but I average about 3.5 days year-round. If I don't set my goal at riding nearly every day, it's too easy to talk myself out of it when I just feel like sleeping in. However, I generally don't ride when the weather is bad (heavy rain, snow, sleet, etc.), when air quality is poor, when I have errands to run or need my car at work.

Bike commuting every day is hard. The varying weather conditions can be tough, and the wear-and-tear on your body can take its toll. That's one reason why I don't feel bad about driving on rainy days, etc., because I figure those are my recovery days. I also take advantage of my driving days to bring clean clothes and food to my office and bring home dirty laundry.