Why is missing Mass a serious sin if it affects only me and God?

"It affects only me and God"? And God? Since when are human persons more important than God? It makes an infinite difference whether we measure God by our standards or ourselves by his. We must start with God: since he came first, he comes first. He is not just a bigger version of us. He has commanded us to keep holy the Lord's day, and he has the right to demand it. Everything of value in our lives, life itself included, has value only because of his infinitely greater value.

Mass is more than acknowledging his sovereignty, which is what worship is. It is being present at the foot of the cross and giving thanks for the Passion and death he endured for us. He loved us so much that he suffered all that to show his love in a way we could somewhat grasp. Skipping Mass is one way of turning our backs on that love.