EINSTEIN's VERY FIRST MONUMENTAL INSIGHT ABOUT GENERAL RELATIVITY.

If an object is moving in space at 99% of the speed of light, for every day it spends in space,7 years would have elapsed on Earth.
But why? Why velocity has a close relation with time/mass?

Let's start with a thought experiment. Imagine you are on a train and it is travelling at the speed of light (just a T.E). If you hold a mirror in front of your face, will you see your reflection in a mirror? How could light from your face reach the mirror if the mirror is travelling away from you? But it would be a pretty spooky train if you couldn't see your reflection so Einstein felt this solution wasn't realistic. On the other hand, if you could see your reflection, it would mean light was travelling at the speed of light inside the train.
But that meant the same light observed from outside the train would be going twice the speed of light. This again seems inconsistent. So Einstein resolved that you must see your reflection but that light must travel at the same speed inside and outside the train. The only way this is possible is if space and time are perceived differently by observers inside and outside the train.
Now here is the point, if space and time are perceived differently by 2 observers then what will happen?
1: We all knew a term that- 'Matter tells space how to curve space tells matter how to move'.

And here the language of their conversation is MASS. And gravity is just curvature on space. So, this imaginary travelling train would curve space more than what it would do when it was in a rest position.
That means it will increase your mass-energy along with train itself. Which would be perceived by an outer observer as yours and train's relativistic mass has increased. This is why relativity of mass occurs.

2: Time starts flowing slowly and everything on the train is in slow motion relative to the rest of the world/observer, just like near the black hole or a massive body. This is because our universe is strict to protect the speed limit. When we move in space, we move in time as well - and the total speed is the speed of light. When speed of that train approaches the speed of light, time slows down - and the total speed in space + in time is maintained at speed of light.

This is known as TIME DILATION.

BELIEVE ME OR NOT THAT IS ALL THAT EINSTEIN GOT IN HIS VERY FIRST MONUMENTAL INSIGHT.