Since wizards, bards, and druids tend to be in the back of the combat line when it comes to the order of combat, they might be standing next to some of their allies. If those allies are within the 15-foot square when the spell Thunderwave is cast, then they too must make the constitution save and take damage. They will also get pushed 10 feet as well if they fail the save, which can really mess up a battle plan!! So if you are standing directly in the middle of your allies, maybe you shouldn’t cast this spell until everyone gets clear. Even if you want to propel an ally an extra ten feet towards an enemy, you probably won’t be greeted with too much enthusiasm for the idea thanks to all the extra damage that can happen.