From Envirowiki

Bicycles are a great way to help the environment. Why? Because they don't pollute (much), they're small and fast, and they can teach you about how important it is to take care of things.

Caring for the environment and other people and animals around us is really just a subset of caring. Looking after your bike properly is part of it too. I can't say it as well as Pirsig...

[edit] Bicycle Activism

Bicycles can be used as a great way to show your support for bicycling. Critical Mass bicycle rides started in the 90's in San Fransisco, and now occur in many cities worldwide. Critical Mass is a fun, positive, free 'push-bike parade' to promote cycling's many virtues. Even though we can be active and bicycle down the road safely doesn't stop motorists not understanding why bikes are better than cars.

[edit] Safe Bicycling

Learning to ride safely doesn't stop when your training wheels come off. Bicycle skills are gained with experience and riding in traffic safely is important. Car culture and societies expectations and views of cyclists has a big impact on the level of bicycling.

[edit] not all flowers

be careful not to fall into the trap of "all bikes are good" - bikes are still, unless great care is taken, consumer goods. the majority of bikes bought new in shops are made with sweatshop labour, in countries like China, and Taiwan, where workers have very few enforced rights. Also, many bikes are not built to last, and the embodied energy in the iron smelting, construction and international transport should be a consideration. This is ofcourse, a much more serious problem in other forms of transport, such as cars, and is not such a problem in second hand bikes.