There is an interesting analogy which came to my mind. Let's say we have $100 each and both decided to buy stock, only I bought e.g. Microsoft shares, and you bought Facebook. Who has more money (i.e. cash)? Hard to tell until we both sell and compare. Similar things happen with inertial frames of references. You can only compare time when you are at the same point in space and have zero relative speed (reminds me of Back To The Future moment when Doc Brown compared watches). E.g. time on the ISS is ticking slower. Really? How do we know that? We put one clock on the ISS (i.e. launch it there), wait some time and bring it back. Yes, it shows less ticks. Is it because ISS is moving? Not really, ISS is following its world line and is roughly an inertial system, so its time is proper. And proper time is always the shortest. We on Earth on the other hand are constantly crossing world lines (unless falling down with 1g of acceleration). Why is it in reverse then? Because we need to accelerate to get there and decelerate returning back. Now imagine that somebody from ISS sends one of their clocks down to the Earth then gets it back. Which clock has less ticks? You guessed it, the one that traveled to Earth and back, because it's our time that is dilated, not their.