Why? Because I like it, that's why. I enjoy it so much more than sitting in a stuffy tube carriage and trying to avoid eye contact with the guy sitting in front of me, or worse still, standing for 45 minutes in my high heels, holding on to the rails for dear life and twisting my neck to keep away from someone's smelly armpits. Honestly, if you live in London, cycling is the only way to remain sane. Even with all those shouting cabbies, bus drivers trying to push you into the kerb and aggressive drivers who think they're always right, cycling is so much more pleasant than using public transport.
A car was never an option - first of all, I don't even have a driver's licence, and secondly, there's many nicer things to do than being stuck in traffic.
Exercise is important, too. I go to the gym 3 times a week, I swim once a week, so I'd still be OK without cycling, but I'd miss it a lot.
Oh, and saving time is vital, too. I save around 30 minutes a day - enough to have a cup of tea, read a chapter of a book or have a chat with my sis on the phone.