Questions: Why can't you reach the horizon? Why can't you reach the end of a rainbow? Why can't you reach the border of Earth?

Answers: Because they don't exist. You can't reach the horizon because by definition, the horizon is something that only you see (it's “your” horizon), and when you move towards it, it moves backwards at the same time, therefore it can never be reached. So it's an illusion just like the end of a rainbow, and you can't reach an illusion, as it's just a physical consequence on your sense of perception, of a non-physical phenomenon.

So, to answer the original question now: why can't you reach the speed of light? Because when you move at 10 km/h as compared to a friend of yours who is not moving as compared to Earth, you can measure that light as you see it is moving at the “speed of light” and that is a very precise constant value, AND your friend can do the measurement of the speed of the light he sees and he will agree on the exact same value. So who is right? And does that mean that light in your reference frame is faster of an extra 10 km/h as compared to his?

The answer is NO, it doesn't mean that. The speed of light is always a constant, whatever the reference frame you choose. In fact, that there is a speed limit is the simple consequence of the fact that “movement is like nothing”, as there is no preferred reference for the measurement of positions and speed in the universe, as a position is always a measurement between two bodies, but which one is to be considered “still”? The answer is: any will do. And because anything can be considered “still”, then all the rules of motion have to be invariant with a translation of the reference, hence Einstein’s equivalence principle.

So what happens when you move as compared to someone else? Well, your personal clock just slows down as compared to his clock, and that is true in order to achieve the invariance principle. So a consequence of that is that the more you accelerate, the more your clocks slows down and the more energy you need to continue to accelerate. Then the ultimate speed (light) is an asymptote on that equation, which means it never can be reached, unless you have infinite energy and infinite time to accelerate.

That ultimate speed is the speed that massless particules have, and it can be shown that they can't move slower, and that they have no internal clock: time doesn't flow for them. So in a way they are “still”, and they can't move in space because they can't move in time, they just teleport and experience the initial conditions of their trip and the ending conditions at the same time and the same place, and by doing so they link those places through space and time.

When you have a mass it costs infinite energy to reach the speed of light, but when you have no mass you reach it immediately and can't move slower. So why is that? What is mass?

Well, mass is the cause of the flow of time. When you have a mass you see a flow of time, when you have no mass you see no flow and no time. And the more massive you are the more time you can experience, that's why in a black hole you experience so much time there that you can't interact with the outside world anymore, it has no time for you, and with no time it doesn't exist for you.

So from the inside of a black hole there is no outside, and from the outside there is no inside, so a black hole isn't an object of our universe, it's a border of it. It's speculated that our universe is infinite, but this doesn't mean that is has no borders. For example the Earth has no borders “on the ground” but the Earth is not infinite, it's finite, it has in 2D what is called “a curvature”. So is it possible that our 3D space has a curvature as well? For a long time in recent history is was speculated that there could be a curvature, but the more we tried to measure it the more we were confident in the fact that we live in a “flat” universe.

But we also know that space-time is curved, and that mass is causing this curvature, so we could conclude that mass is everywhere the border of an empty universe… In other words, mass would be the shape of the universe, that's why it's the only “real” thing that there is.