1. You're correct, you work with a moron but maybe

2. he's insecure and jealous of you so he feels the need to point it out when you don't ride? also

3. I, personally, reject "purists" who think you're not a "real" cyclist or whatever if you don't ride everyday. To me, commuting is about having fun and staying healthy. If you can't have fun and be safe then that's YOUR call, no one else's. No shame in not riding on ANY day you see fit for ANY reason.