Why does my baby keep pulling on her ears?

Pleasure or pain. Babies pull, rub, scratch, and grab everything they can get their hands on. Most of the time they are exploring and testing new 'toys.' sometimes, though, it can be a sign of discomfort. If she is otherwise happy, playful, and doing her normal things, she may have a little extra wax or just having fun. If she is fussy, she may have ear or jaw pain, so that would be a good reason to see her pcp.
Teething. If the ear pulling is not associated with fever or a bad cold, it is highly unlikely for this to be due to an ear infection. Often teething pain can be referred to the ear as ear pain or the baby may have just "found" his or her ears and they like to play with them. Fluid behind the ear drum from a recent cold could also be the culprit. If it persists or is accompanied by fever, call the doc.
Because she can! Babies pull, tug, and play with their ears usually because they can. They have discovered these interesting appendages attached to the side of their heads and they can't resist. Sometimes, infants who are teething will pull and tug at their ears. Ear pulling in the absence of other symptoms is rarely associated with ear infections.