1. There's no such thing as "sin," it's something made up in order to control the actions of people. Some of the reasons are good, mostly from my estimate, they're bad.
2. The reason is to control people, as said in reason one...
But if you're talking about a hypothetical world, where people are born into something that they have to make up for later by being beholden to some power, or putting change in the basket they pass around, or feeling really guilty and saying prayers to an invisible man in the sky, or having water poored on their head -
In that case, human's were born into sin because we have desires.
Science would call this instinct, but -- procreate with anyone you're attracted to (married or not,) want stuff, want what you see others have, etc. Religion calls it original sin, (or a deviation from original sin, point is, it's natural - can't be helped.)
The mind automatically follows scenerios to their intended result. ie, if you want your neighbor's wife, you'll not only desire her, but you'll play out scenarios in your head that result in your goal, sometimes in a flash, other times over time, other times, humans follow through.
Murder, greed, gluttony... all seven of them.
We can't help thinking about it. That's how the mind works.
Religion argues this is your fault though, even if you don't follow through. (thus original sin)
People like myself, and other more human sympethetic people,
realize that, it's not what you think, but what you do,
that makes you a good person, or a sinful person.
And ultimately the only reward is here on Earth, and what we make of it.
Or maybe we're all going to hell, who knows?
But some would argue, that if one were to be damned for something they cannot help,
perhaps the one doing the damning carries the greater guilt ultimately.
But, religion has big, scary characters. And eternity and death, they are big scary things.
And if a little kneeling now, will save one an eternity of pain,
who would I or you be to tell god he's wrong?
