5. You protect the environment.
Carbon monoxide, sulphur oxides, oxides of nitrogen, hydrocarbons – cars are responsible for most of air pollution in modern cities. Bikes, on the other hand, produce zero harmful substances (and building a bike only takes about 5% of the materials and energy used to make a car.)
Air pollution causes all sorts of unpleasant health issues: some studies claim it is responsible for twice as many deaths as traffic accidents. Sure, buying a bike will not create a pollution-proof bubble around you, but at least you will be breathing a little less exhaust on a daily basis. And who knows: maybe you could convince some other people to ditch their cars and join you on your environmental crusade as well?