Why must the positive wire go on side of a battery?

This statement isn't always correct. There is no inherent characteristic of a battery that causes one of its terminals to be positive and the other negative. For example, car batteries are most commonly marked as having positive and negative terminals, but if you examine them, you'll see that they're identical. If you connect a voltmeter to the battery, it will tell you which terminal is positive and which is negative, but this is just an indication of the current flow direction in the circuit, which can be reversed by simply swapping the wires (and reversing the current flow).

A battery creates an electric field in the space around it, and the positive terminal is the point where the electric field lines point away from the battery, while the negative terminal is the point where the electric field lines point toward the battery. Electric current is defined as the flow of positive charge carriers in an electric field, so conventionally we assign positive charge to the terminal with the higher electric potential (where the electric field lines point away from the battery).

The convention that the positive terminal is red and the negative terminal is black is also not universal, so for example, in certain types of batteries (e.g. nickel-cadmium batteries), the terminals are red and blue.