I cycle in to work because:
The wildlife. Today I saw Peacocks (yes, wild), a stag, countless birdies that I don't recognise, one flying alongside me which is kinda cool.
The social life. I get to say hello to people on my commute I wouldn't have even recognised if I went by car. This morning I saw an old work colleague. He berated me for still being slow.

I also nod hello to joggers, other random cyclists, people walking their dogs... the odd thing is I occasionally see these people later on, and we chat like old friends.
The cost - yes its cheaper than petrol, and parking, and wear and tear on the car. On the subject of cars, the last time I drove to work my car was boxed in by some cretins, and the Police couldn't care less. I ended up, um, carefully manoeuvring my way out.
I can arrive alert and attentive... and usually earlier than I would arrive by car. As I work flexitime that means I leave early too!
I get to see some fantastic scenery, sunrises, sunsets, mists and so on.
People are usually impressed - its a good conversation starter, especially with the top brass at work.
A greater understanding of the seasons.
Better road sense.
Is that enough? There is more but its getting kinda listy.