Lightspeed is the same to all observers.
All rest frames are equally valid.
Look at it this way; when you're going at 30mph, what are you doing it relative to?
The planet.
However, it's equally valid to say that the planet is going 30mph with you at rest (assuming no acceleration occurs).
Another way of looking at it is...
You're in space. You think you're stationary, yes?
Then some other guy drifts past you.
To HIM, it feels like he is stationary, and you are drifting past HIM.