The PFSRD says this on summoners:
While many who dabble in the arcane become adept at beckoning monsters from the farthest reaches of the planes, none are more skilled at it than the summoner. This practitioner of the arcane arts forms a close bond with one particular outsider, known as an eidolon, who gains power as the summoner becomes more proficient at his summoning. Over time, the two become linked, eventually even sharing a shard of the same soul. But this power comes with a price: the summoner’s spells and abilities are limited due to his time spent enhancing the power and exploring the nature of his eidolon.
Notably, there's no hard and fast way that this happens, this is up to the player to fill in.
This is as much to fill in as what started a wizard down that class path.
Some good reasons why this MAY have happened, can vary. Perhaps your character was messing around with summoning spells as a child, and due to some mis-step, it persisted, and sent them down the route.
They might decide they had native magical abilities, and had an eidolon as a lifelong imaginary friend, who happened to appear to other people one day.
or they could have enrolled in some kind of summoning school that their rich parents sent them to, in an attempt to have their child be a magical town guard.
It's entirely based on the character's interpretation, but as mxyzplk points out in this answer, perhaps you want to base it in a specific location's setting, relating to Golarion.
As a DM you can always write in that a local town had a summoning school, or a strong link to a different plane, as you see fit, but the setting is up to you!