On certain levels, this is a simple question to answer: Our mommies and our daddies liked each other very much, and they shared a special kind of hug, and then a few months later, along we came! Simple, but as my friends who've explained this matter to their children have told me, not necessarily easy.
But if you spool back a ways, you can get to a place where that's a question without a good answer yet. The elements that make up our bodies are what's left over from the explosions of the first stars. The first stars condensed out of the primordial galactic particle soup that came from the big bang. Where that came from, we don't really know, but that's not the mystery of why we're around.
Every subatomic particle has an "anti-particle" that is like it in every detail except for having the opposite electrical charge and a goatee. One of those may not be true.
Of course, the designations of "particle" and "anti-particle" are arbitrary, just like the ideas of "positive" and "negative" charges. The key is that they're opposites of each other, and whenever they encounter each other they annihilate in a burst of energy. This is why there aren't anti-particle people walking around exploding when they touch their counterparts.
Which leads us to our mystery. Since there's no preference in particle creation, the big bang should have made exactly the same amount of matter and antimatter. They should have subsequently wiped each other out and left an empty universe. Our presence seems to suggest this did not happen, so we know the amount of matter in the early universe exceeded the amount of antimatter.
What we don't know, and what a lot of physicists and cosmologists are engaged in trying to find out, is why it did, because it shouldn't have. Scientists are the people who respond to the suggestion about not looking a gift horse in the mouth with a "why wouldn't you?" The researchers in the story at the link are following clues suggested by particles called "beauty quark" and "heavy neutrino."
Expect some silly undergraduate to start complaining about these labels promoting "lookism" at some point in the future, when we will wish for some anti-particles to appear and annihilate them. But in order to be true anti-particles to people this dumb, they will be way too smart to be tricked into that. On the other hand, they'll probably also be smart enough to figure out the answer to the imbalance.