Reference phasor(or quantity)
Why is the reference always 0.


Because it makes the maths easier.

For voltage measurements we choose some part of the circuit as reference and call it zero. All voltage measurements can be taken with reference to that point.

In the same way a surveyor takes some height reference and all subsequent height measurements are + or - relative to that point. Sea-level is an obvious choice but might not make much sense for a building at 3,000 m above sea level where a local reference can be chosen.

For your phasors you take one as the reference and everything else leads or lags that.

You can, of course, use a non-zero reference if it suits. This might be the case if, for example, the system is referenced to another (such as the mains) by a fixed offset. It may suit to do all the calculations relative to the mains even though it's not relevant to the immediate calculation.