Why it is lucky cutting hair when the moon is waxing?

The superstition concerning the cutting of hair states that cutting your hair (and nails) during a waning (growing smaller) moon will keep the hair (and nails) from growing out as fast or as strong, and will impact your health adversely; cutting the hair (and nails) during a waxing (growing larger) moon will make the hair (and nails) grow out stronger, and faster as well as having a beneficial effect on your health.

There is no proviso on luck in the superstition.