Every digit-reduction system has to have a starting number, and in base-10 arithmetic that's 1 — the smallest nonzero digit you can reduce down to. Numerology took that structural fact and built a whole character sketch on top of it.
Because any birth date that ultimately reduces to 1 has, by definition, produced a total like 10, 19, 28, 37, 46, or similar, the "1" theme in numerology draws heavily on the symbolism of standing alone at the front of a sequence — the number before which there's nothing, and after which everything else follows.
In Vedic-influenced numerology this number is sometimes linked to the Sun, treated as the source that other planets orbit — reinforcing the same "center of things" framing that Western Pythagorean tradition arrives at independently through the idea of unity and origin.
