But we must be cautious about what life we spread. If we fill a planet with wildlife similar to earth's, then we are morally responsible for all of the suffering experienced by all life on that planet. There is also the possibility that lifeforms on this planet will evolve into something hostile, that will try to attack other planets.
We don't want to just spread life for the sake of spreading life. We want to fill the universe with beings living happy and fulfilling lives.