Time, space, and velocity in our expanding Universe

Why does the relative velocity between two frames of reference cause the length and time variations predicted by the Lorentz transformations? We know that a mass causes length contraction and time dilation in its surrounding spacetime. Could gravity be the cause of the length and time variations due to velocity?

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With thanks to NASA, ESA, G. Illingworth (University of California, Santa Cruz), and G. Bacon (STScI) for the Hubble Legacy Field – Zoom of the galaxies, and the Hubble telescope’s deep field pictures on the home page, and the other pages on this site.