I rarely smile at other drivers while in my car. Sure, if I know someone, I wave and smile. Otherwise, I'm ambivalent or cursing under my breath, depending on other drivers.
When I'm on my bike, I often get smiles and/or waves from other people sharing the road - other parents riding, people walking and sometimes people driving. Today, when I rode up to the chain grocery store to buy gift cards for the kids' teachers, a woman smiled and said hello to me. Had I driven and walked into the store, I doubt that we would have shared a look, much less a smile. I've had conversations about my bike, about my bike trailer, about my coat, about riding, about all sorts of random things with people because I ride my bike. And because I work from home and don't interact with many people throughout the day, I value being able to share a little with people. And if it's because I ride my bike, all the better.
I can't write about interactions without mentioning my social media interactions that have occurred because of #everydayonabike. I now follow many people who are just like me - not serious bikers, but like to commute. It's been great to "meet" people through Instagram and the blog who share a passion for biking.