Two hundred million sperms raced for an egg, and somehow, 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. If your father had woken up one more morning for a walk and upon returning saw your mother shining like a goddess in the light coming through the window. Now consider the odds of the birth of the right ancestor and their whole life, odds ranging from something as miniscule as the same time of urination to something as big as marrying the right person. Something like Dr. Manhattan's quote from comics “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. What do you think will happen otherwise? Alternate timeline, timeline branching? 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. Same house, same school. And 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 consciousness. It couldn't be of your best friend, couldn't be of your neighbor, well, couldn't be of any person you've ever seen. Maybe 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 steering the boat.
You see, nothing carries consciousness. Well the sperm certainly doesn't carry that, just chromosomes and other fluid stuff. And certainly, consciousness cannot be donated or couldn’t be the same as of 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. Different body, but you.
Now you may be thinking, what if, let's say, a different sperm was fused inside my grandmother's womb? My father wouldn’t exist that way. Okay. Let's say a girl was born instead. 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”, 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 just have different thoughts, different opinions, 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. 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.