In the winter time it seems that everyone is shocking each other. I shock myself on every chair at school, I swear. The worst feeling however, is when I shock my cat. Most of the time it happens when I'm petting her.
The reason why is that when I pet her, I am picking up electrons from her. This gives her a net positive charge and myself a net negative charge. I don't know the magnitude of the charges but we would have equal and opposite charges, assuming I am not grounded and my body contains the electrons, the same with her.
When I go to pet her again, our net charges come into contact, and the shock comes from the electrons "jumping" back to her fur and leaving my body.
The same sort of thing can happen when you are wearing fuzzy socks or slippers. When you walk, you scrape electrons off of the fabric, and when you go to touch someone, the excess electrons jump onto them in the form of the shock.
Likely, part of the reason why you seem to shock more people during the winter is that you wear warmer shoes or socks during the winter, and these items act as better insulators, preventing the net charge on your body from leaving to the Earth.