Why is the gospel side of the altar the left side?

In the post-Vatican II Church, there is no such differentiation. In the older Church, when you faced the altar the Gospel side was to the left and the Epistle side to the right. I have found no particular significance to this arrangement. It seems to be a custom handed down for hundreds of years.

Personal reflection

All altars used to face east. When you are standing facing the altar, the left was the north, and the south was on your right. I was told that the altar boy carrying the book from the Epistle side (right or south) to the Gospel side (left or north) was to symbolize the Gospel being carried north from Italy and the Mediterranean through Europe the further north. I don't know if this was something they came up with after the fact or how it came to be. However it came to be, it is referred to as the Gospel side because that it is the side where the book and the priest are when the Gospel is proclaimed, in the traditional format of the Mass