Answer:
It is necessary to do the same thing on both sides of an equation in order for the both sides to remain equal. If only one side was changed, the equation would not be equal anymore. When the same operations for values are done on both sides, both sides are increasing or decreasing the same amount.
Metaphor:
Two sides of an equation are like two sides on a balance scale. If you put things on one side and not the other, one side would sink lower than the other. The unbalanced scale is not parallel to the ground and unequal.
Analogy:
If I only give your friend chocolate and none to you, that's unfair, unequal.