To be merry is to be very happy, and in the mood to celebrate. At Christmas time, people say “Merry Christmas!” to wish others a happy holiday season.
Having more of some things in your life, like a cold, or a flat tire, might not make you happy. But how about having more friends, or, if you’re an animal lover, more pets? If you’re having a small party and another friend asks if she can join, you might tell her, “Sure! The more the merrier!”