I bike to work because when I made the switch my commute went from a frustration to the highlight of my day. I love to ride and dont get a lot of chances with work, kids, and other responsibilities. Commuting helps keep me from going crazy.

Because I can.
I would guess it's the last rebel thing I could do, and not get into any big problems.

That and it's fun being car free!
I love the way people look at you funny when you tell them you don't own a car.

At first it was to lose weight, then I got in pretty good shape and now I'm in love with biking. There's also gas price, since I commute 61 kms every day I save about 55$/week wich I usually spend on biking gear, or bike tools or bike parts.

I commute because I like to be part of the world instead of hidden from it. There's so much more to see on a bike.

Best way to get in riding time without impacting family time.

Because it is fun to ride a bike. Very beautiful scenery. Exercise.
I ride in good weather, sometimes even when unsure about forecast.
I hope the get a fenderable bike so I can ride even in the rain.
Haven't figured out how to do winter yet, though.

I have a scooter, a motorcycle, a car, and 5 bikes. I don't use the car ever, the motorcycle gets used for days when time makes it impossible to ride a bike - maybe once a week if that. I commute on my bike because its the most efficient way too:

1) Create less carbon dioxide
2) Add to my weekly exercise milage.
3) Make me feel good when I get to work
4) See the sights as I pass
5) Because its just plain FUN.

I hate cars. 2 tonne to move 90kg of person? Totally inefficient. If I have my wife, my 2 kids, and luggage as well.. then I tolerate it. Since the price of fuel keeps going up the motorcycle plain costs too much to run. The scooter is so slow I can almost go faster on my mtb so its relegated to parking blocker duty. I ride a cruiser style motorcycle in the winter even if it snows so I figure one of the older mtbs will do fine for that.

Also a $1000 bike is wayyyyyyyyyyyyyyyy better than a $1000 car

As globie suggested, commuting to work just adds five extra days to my chances to be on a bicycle. Why feel so alive for just two days a week?

For me
I found out that I have to pay big time in taxes and on a payment plan so I had to figure out how to save money. I love to ride and now I dont ever hear my self say "I NEED TO GO RIDING" So with the diesel truck parked I am killing 3 birds with one bike.
Exercise
save money
and enjoy it

I started primarily for environmental reasons, and that's more than half the reason I continue. But it also keeps my blood pressure down better than medication. I have no doubt that it's adding years of healthy living to my lifespan. And it's fun, and when I get to where I'm going, I feel great and full of energy.

When I don't ride, I feel like a slug all day, I'm not very productive, and after several days of not riding my blood pressure is significantly elevated.

it's the most logical choice.

and yes for me it's a choice (I have a license and a car, choose to use neither whenever possible)

1. Sounded like a fun challenge.
2. Wanted to recapture the joy I felt as a teenager on a bike.

Succeeded x2. Never looked back -- in fact, not long after that, I let my then-estranged wife use the family van during the day when I dropped my daughter off on the way to work, pulled the bike out of the van, and rolled. Two years later, the last car I owned went belly-up; I junked it, got back on the bike, and rolled.

Not for me...my wife doesn't like when I take longer rides, even though everyone benefits from me riding ( less irritable).

I need to get to work.

1. They won't pay be to stay home.
2. I refuse to own a car or get a driver's licence.

Commuting to work with my bicycle is my motivation to go to work, otherwise I find myself calling in Sick way to often.

Nice Jack Casady bass!!

I get to ride while the wife is either sleeping or already at work. That's what I call minimal impact on family time....perfect.

Besides the other obvious benefits of commuting of course.

It started as a hobby and to lose weight just over two years ago. I would ride 4-5 miles a day, and work on pedaling for as long as I could without stopping. After about 2 months I realized I may just be able to pull that 20 mile round trip commute to work over the summer. So I did, and eventually I built up to getting 10 miles in 35-40 minutes consistently on my (since traded) Trek Soho S. My wife complains about how I jump from bike to bike, but hey, I just love all kinds and she will only let me have one at a time. Anywhosit. I've ridden 3000 miles since I started, and I'm not looking back. But yeah lose weight, have fun, save money on gas/insurance/car payment/repairs. Plus that urban hipster thing HAHA I too have recently been visiting my local farmers market, BY BIKE!

In order:

Fitness/Weight Loss
Mental Clearing
Save some gas money so I can spend more money on stuff for commuting so I can save some gas money so I can spend more ...... etc

The wind flowing past, the freedom riding straight and fast, or slow and curvy, mashing uphill, and gliding down, the feel of my heart, the rush of adrenaline that lasts all day till the ride home, the cool misty mornings, the bright sunny days, the places I see in more detail, the people I see every day and get to know, that I would never pay any attention to in a car, the challenge of packing gear, the pleasure of beating the system, the pride in doing something clean and joyful, the rain on my face in my eyes soaking my clothes knowing that it's ok to get wet, breaking free of the chains of convention and realizing that I can do so much more than I ever realized.

1. Exercise, good cardio
2. It's fun, and cool
3. I hate driving, in general
4. I don't want to be depended on cars for transportation
5. To brag of the vast distances I can travel with my own legs on a bike

I originally started to ride because of environmental reasons (i.e, freaking out because of global warming). I've continued to be concerned about the environment, but in all honesty I continue to ride mostly because driving a car turns me into an angry impatient *******. When I ride, I'm happy.

Too lazy to walk; too antisocial to take public transit; too cheap to pay for parking.