Being warm and comfortable definitely keeps me motivated... I used to just NOT ride my bike when it rained (although I had no problem with snow/subfreezing temperatures). Once I tried it a few times with appropriate clothing, I found it was fine.

I think my biggest motivation to bike to work and wherever I can is that I feel so much better. I just feel energetic and cheerful all day. And I can skip out of work and go on a ride if I want =)