Question

Why potential at infinity is always taken zero ?

Answer 1

Because any field is measured radially with an inverse square relationship of intensity to distance. This means that the measure of, for example potential, is proportional to 1/r² so no matter how far you are from the field there is still some influence from it which decreases exponentially with the distance (radius).

The electric potential is taken from infinity as the only time the fraction 1/x is zero is when x equals ∞, as the fraction tends towards zero and the denominator tends towards infinity