Sorry again for the late reply, i promised a post the following day, but i had to travel for family reasons.
Anyway, the basics of your question are, why is time relative and not a constant? It is an extremely complicated question and though your right that time slows at high speeds relative to someone at low speed, it is not the only instance of relative times. The other case is when in the presence of a gravitational well. In other words near something very large (planet). The closer you are to a large gravitational body the slower time moves.
This is all summed up in Einsteins theories of relativity, there are other theories which expand further and could be more accurate but Einsteins are the simplest and extremely consistent with most physics experiments.
The best way to explain this is use the postulates of special relativity.
So we start with two postulates (something we take as fact when developing a theory).
The laws of physics are invariant in all inertial systems, ie systems not undergoing acceleration
( https://en.wikipedia.org/wiki/Inertial_frame_of_reference)The speed of light in a vacuum is the same for all observers.
Now this may not be obvious to us but this gave a very clear problem to Einstein. If the speed of light is constant then what happens if the light source moves? The speed of light is a physical constant and cant be changed, so there is a problem. You move a lit torch forwards and backwards which would imply that the speed of the light gets larger and smaller. However it cant. So we have three solutions, either time, distance or both are changing.
We can show this with the "light on a moving train" scenario.
So we have a pulse of light bouncing between two mirrors it always travels the same distance ( L ) in the same time ( t ) as the speed of light ( c ) is constant.
We now put this system on a moving train that's moving with constant velocity ( v ). To someone on the train the system is exactly the same. However to someone observing from a platform as the train goes by, they see something different.
We can see that the distance travelled by the light pulse is longer. They see the light travel a distance D not L.
So we have this pulse of light travelling two different distances depending on who's observing it. This differing distance means there are differing times to complete them.
We consider how long the light takes to complete a loop from one mirror off the other and back to the first we get.
t1 = 2L/c for the observer on the train t2=2D/C for the observer on the platform
The result of this is that to the person on the platform the time taken to complete a loop is longer as the distance the light travels is longer. So the person on the platform is experiencing more time than the person on the train.
Thinks now get very complicated in the fact that there is no such thing as universal time distance or velocity. ALL of theses are relative, so who is moving? Who is stationary. The answer is neither, and both. They are moving relative to each other, you can not say who is "moving" and who is "stationary".
So the observer on the train sees the person on the platforms time move at a slower rate and the person on the platform sees the person on the trains time move at a slower rate.
They both observe the same phenomenon, this opens a brand new box of complicated questions and equations but the root of the problem is solved by the fact that in order for these two people to see each other again and measure who experienced more time one of them has to undergo acceleration (eg turning around). This defines which one experiences more time.
Now i realise that this still doesnt in all ways answer the question why?
The truth is that we look at the world wrong. We see a three dimensional world of space (3D) and we experience time. We feel that these are constants and never changing. A meter is a meter and a second is a second.
Minkowski space (http://mathworld.wolfram.com/MinkowskiSpace.html) says this is not the case. What we consider are two separate things are actually part of something whole "The Space-time Continuum".
So what does that mean? Space-time is constant and has intervals that are constant. eg similar to what we believed time to be like, seconds, minutes hours, etc. We are therefore all constantly travelling through space-time at the same rate, we all experience the same amount of space-time.
Now it gets rather beautiful!
It turns out there is a universal speed limit and it defines how much space-time we all travel through in a given "time" (this time is not normal time im using it illustrate a interval of space-time). Everything travels through the same amount of space-time relative to eachother (constant). So space-time is made out of space and time so we can choose how much of each to travel through. You travel through more space, you experience less time and vise versa. You could be stationary and so spend all your space-time going through time. Therefore stationary observers experience the most time.
Finally it is commonly believed that the speed through space-time is in fact the speed of light and that the speed of light is not a property of light, rather a consequence of having no mass. So with no mass you travel at the universal speed limit through space-time. This can help explain why the speed of light is constant in a vacuum.
It also means that light is travelling through maximum space from our reference frame, which implies it travels through no time at all. From the photons perspective it travels through no time and no space at all. Now limit sets the reason why time travel is impossible. Light travels at the maximum speed and travels the maximum distance in space and experiences no time at all. So the only way to go back in time would be to travel faster than the speed of light, which as we know is impossible as it is the speed we travel through the constant of the space-time continuum.
I hope that helped answer your question.
If you have any further questions or need more detailed or simplified explanations please don't hesitate to write in the comments section.
The Surf Graduate