I'm the same as you: in cold weather all I can think of on the way to work is how much fun commuting ISN'T. When it warms up- my whole attitude changes. I think for me it has something to do with the darkness/cold combination. When it warms up, it also lightens up which makes my mood much better.
I'm glad you had a great ride this morning, and sleeveless in the afternoon sounds simply wonderful! Enjoy the ride home!