Two hundred million sperms raced for an egg and it was you who came out. Just a little bit of tweaking from your parents could have changed that. Say if your parents had married a day earlier, or a day later. Or if one more day your father had woken up early and gone for a morning walk and when he came back inside the room, he saw your mother shining like a goddess in the light coming through the window. Now consider the odds of the right ancestor being born and living and marrying the right person, the odds of all things being precise till you were born…something like Dr. Manhattan's quote from The Watchmen by Alan Moore.
Thermodynamic miracles... events with odds against so astronomical they're effectively impossible, like oxygen spontaneously becoming gold. I long to observe such a thing.
And yet, in each human coupling, a thousand million sperm vie for a single egg. Multiply those odds by countless generations, against the odds of your ancestors being alive; meeting; siring this precise son; that exact daughter... Until your mother loves a man she has every reason to hate, and of that union, of the thousand million children competing for fertilization, it was you, only you, that emerged. To distill so specific a form from that chaos of improbability, like turning air to gold... that is the crowning unlikelihood. The thermodynamic miracle.

But it's always going to be you. Only you. There is no one else. Let's say a different sperm is fused—and, what do you think will happen? Alternate timeline, timeline branching? In that universe things will definitely be different. Let's say, you are a son here—and there, a daughter is born. And maybe she will have more grades than what you've got here, or maybe she will be much prettier and so on. But still, it's going to be you. I mean the “you” you.
Let me elaborate. So “you”, the parallel universe's one, will of course have the same parents as you have here. Furthermore, many things will be similar. Same house, same school. But think about it, if someone lives in your house, in your room, and is the child of your very own parents, who do you think that will be? I don't mean the body. I don’t mean anything physical. I mean the reasoning, the consciousness. It couldn't be your best friend, couldn't be your neighbor, well, couldn't be any person you've ever seen. Maybe an unknown or an imaginary person, or precisely, an unknown or an imaginary consciousness. But no, the most likely candidate is “you”. Just a different body, different hair length, different voice, different friends, and so on, but inside, the same consciousness behind the wheel.
You see, nothing carries consciousness. Well the sperm certainly doesn't carry that, just DNA and other fluid stuff. Many people don't get the feeling of self-awareness until around the age of four. One day, children just feel that they exist, that they are here. They feel the “I”. And certainly, consciousness cannot be donated or couldn’t be the same as anyone else. So, even if you were born a day earlier or a day later than your current birthday, someone will of course live in your room, someone will of course greet the same mother, same father. Someone will probably use the same internet and if he or she ever stumbles across this article, that someone will always be you. It will be “you” who reads my words. Different body, but you.
And if, let's say a different sperm got fused inside your grandmother's womb, so your father doesn't exist. Let's say a girl was born. So no connection with your mom, or even if there was, and if they ever think of adopting a baby, who do you think that will be? No, not “you” silly, this time.

But does that mean you—and I mean that “you” doesn't exist in that world? Like, really? How can you imagine that? The body might not be the same as your present form. You might have different thoughts, different opinions, different histories, and different childhood, but how can there not be something that has the “I” that belongs to you at present? Think about it. “You” can't just not exist, ever, anywhere—maybe, even after your present form dies. You might just wake up somewhere, in a body that was in an autopilot mode but filled with memories and histories, and the knowledge of how it has come to this exact point and what it needs to do now. But still, it will be “you” on the inside, driving it. Just like how your consciousness comes back and takes over when you wake up from a deep sleep in the middle of the night, whether just to urinate, or to start your day.