This question already has an answer here:
Consider the following scenario:
I get in a spaceship, and travel really close to the speed of light for a while, and then come back.
A lot of time has passed on the Earth, but since I was traveling so fast, I only experienced a few years passing.
So, my friends on Earth are dead, whereas I'm only a few years older.
But what I'm having trouble wrapping my head around, is why is it them that's dead, and not me?
After all, given what I understand about relativity, it's just as fair to say that my spaceship stayed still, and it was actually the Earth that traveled really fast and then came back to my ship.
In that scenario though, the Earth being the fast-moving ship, and my ship being the stationary body, wouldn't it be that I am dead, and everyone on the Earth is just a few years older?
If there really is no preferred frame of reference, then why does the ship-traveler live while the people on the Earth die?