From relativity, if a rocket moves close to c, then its mass increases along with everything inside the rocket. Now what if there was a scale inside the rocket? Would the scale read a higher value? I assume the scale should stay the same because if it didn't, then you can do an experiment to determine whether you are moving or not, and being able to detect a moving reference frame is not allowed. So the true question should be, why would the scale read the same number even though mass is being increased?