It is a generally accepted Scrum dictum that one of the ScrumMaster's duties is to protect the team. The usual example is that the ScrumMaster must protect the team from an overly aggressive product owner. There is nothing wrong with this example and many teams do need to be protected from a product owner whose desire for more functionality can push the team into cutting quality corners.
However, a good ScrumMaster also protects the team from a problem that can be even more harmful: complacency. After achieving some easy, early improvement from Scrum, some teams become self-satisfied. They stop seeking further improvements. It is up to the ScrumMasters of those teams to protect them from this complacency.
So, a good ScrumMaster will occasionally have to stand up to an aggressive product owner and say, “Now is not the time to push this team any harder. They're working as hard as they can and if pushed more, they're likely to get sloppy.” But my advice to good ScrumMasters is that for each time they say this, they should later tell the product owner something like, “Now's the time. The team is rested. They're ready. Let's see what they can do. It's time to push them for more.” If you're a ScrumMaster, be sure to protect your team from both types of problem.