My local LBS has this great display at the front of their shop. It is a manequin, dressed in jeans, a shirt and flannel, with a sign on it which says "This is appropriate cycling clothing". That might seem like an obvious statement to some, but it was that realization that transformed cycling from a recreational sport for me to a mode of transportation.
I have been mountain biking for 15 years, and occasionally rode into work over that time, but it was really three years ago when I built up my fixie that I made the transition to using my bike as my primary mode of transportation. The biggest part to making that happen was realizing that I didn't need a shower at the other end of my ride, I didn't even need a bathroom. I just needed to roll up my pants and get on with it.
I ride to work virtually every day, and although not a particularly long route (~3 miles) it does have 400 feet of climbing in it, and I'm doing it on a 48/16 fixie. And I do it every day in my street clothes and the same shoes I wear all day long. I don't need to wash an extra set of clothes every week because my cycling clothes ARE my street clothes. I don't need to carry any toiletries in my backpack because I don't NEED them, I arrive at work, sit down and get to it.
Once you make that transition, that riding doesn't require extra gear, or extra conveniences, then suddenly your bike becomes your car. You can bike to your friend's house across the city for a party just as you would drive over. You can bike to dates, you can bike to shows, you can bike to the grocery store, all of these without any extra planning or fuss.
Now I admit we might not all work in positions where you can get away with jeans and a t-shirt, but a great many of us do, and a great many more could run their errands this way.
So keep that lycra in the closet now and then and just hop on your bike. You'll be amazed just how doable it is, and how much more frequently you'll find yourself riding instead of driving.