Answer: It is because a baby who begins to perceive the world is basically just a small child. So are we, we are found in a situation like children when we are revealing the upper world.
In other words, when we reach the spiritual holiday of Chanukah, our state is called a state of Katnut (smallness) of the soul—“Galgalta ve Eynaim.”
So in our world we sing with children and eat jelly doughnuts (sufganiyot). In general, the holiday is very happy, soft, and childlike because that is how we begin to reveal the upper world; in spiritual attainment we are still children.
From KabTV’s “News with Michael Laitman” 12/19/16