Answer: Let's discuss on how what might be helpful to humanity, might not be so helpful to other lifeforms. Take for example, we need oxygen to breath, and one the first things we will terraform is the atmosphere, so we can breathe. However, the aliens on the planet breathe nitrogen, and oxygen in large quantities are poisonous to them. Boom, they have been harmed. Next example, an alien species is aquatic, but we need to colonize this water planet because of how perfect it is for our needs. So we cause the land shelves to rise, and create land. Boom, aquatic aliens have reduced habitat, and human pollution will slowly poison the seas as well. (Terraforming will probably pollute a world in some way). It is the same whenever we change the necessary habitat of aliens for our own needs.
TLDR: Human necessities clash with alien necessities.
