Do guys send mixed signals to ALL girls, or just ones they like?

The guy I like will sometimes act really sweet to me, smiling and joking a lot, then suddenly ignore me and act aloof for a few days. Then he'll go back to being friendly again. I've noticed that he definitely doesn't run so "hot and cold" with his guy friends, but I'm not sure if he does it to other girls since I never see him interact with them (I'm the only girl at work). So I'm wondering if this means he has mixed feelings about me, or if this is just something guys do to most girls?