Goku chose to have his tail removed permanently because it was a major liability for him. Firstly, his tail previously triggered his transformation into a Great Ape (Oozaru) when he looked at the full moon, which caused him to loose control and go on destructive rampages. This transformation was very dangerous and harmfull for his friends and moreover Earth.
Additionally, during the World Martial Arts Tournament, Goku’s tail was a weak point that opponents could exploit. So, he chose to remove his tail permanently that made him a more focused and controlled fighter and also he did not harm his surroundings with uncontrollable transformations.
