Why is my Taijiquan Form Lopsided?

I get this question a lot, and not just in Taijiquan. "Why is my form lopsided? Some movements are only done on one side."

Answer 1 (Glib):

It isn't lopsided.

Answer 2 (Useful):

A form isn't necessarily a drill. In a drill, you perform your moves on both sides. A form is designed to hold the information necessary to reconstruct a system.

A lot of systems have forms that are also drills, in that each move is performed on both sides, but this is not a necessary feature of a form.

You are supposed to take each movement from the form and practice it independently, generally on both sides.

Answer 3:

The reason  lot of movements in, for example, the Yang form are practiced only on one side is that the Yang family system defaults to a left lead. You are constantly trying to return to your fighting stance. Therefore, you are less likely to perform Single Whip on the other side.

This doesn't mean that you shouldn't practice it on both sides.