This article is from the Calendars FAQ, by Claus Tondering email@example.com with numerous contributions by others.
There are quite a few theories about this. And many of the theories
are presented as if they were indisputable historical fact.
Here are two theories that I personally consider likely:
1. According to the Gospel of Luke (3:1 & 3:23) Jesus was "about
thirty years old" shortly after "the fifteenth year of the reign of
Tiberius Caesar". Tiberius became emperor in AD 14. If you combine
these numbers you reach a birthyear for Jesus that is strikingly
close to the beginning of our year reckoning. This may have been
the basis for Dionysius' calculations.
2. Dionysius' original task was to calculate an Easter table. In the
Julian calendar, the dates for Easter repeat every 532 years (see
section 2.12.9). The first year in Dionysius' Easter tables is AD
532. Is it a coincidence that the number 532 appears twice here? Or
did Dionysius perhaps fix Jesus' birthyear so that his own Easter
tables would start exactly at the beginning of the second Easter
cycle after Jesus' birth?