Kepler's second law says that planets sweep equal areas in equal times in their orbits; the closer a planet is to its parent body, the faster it moves; the farther it is, the slower it moves. This is explained by the conservation of angular momentum, which is $m * v * r$.
$m$ is the planet's mass
$v$ is the planet's orbital velocity
$r$ is its distance from the parent body
As $r$ decreases, $v$ must increase to maintain the momentum.
Why is $v$ "chosen by the Universe" to be the value that changes in this phenomena and never $m$? I've only heard of mass being modified theoretically, when something is attempting to increase $v$ beyond the speed of light. If that's true, why does mass remain immutable until that point?