If it's a summoner pulling aggro with their tank pet, take a moment to think of a couple of things. They have a very quick casting heal that will keep that pet alive effortlessly over time. That's more damage that you aren't taking, that your healer doesn't have to heal. So if it's a tank pet taking damage, whatever, right? It's just less work for everyone involved. Getting mad at a summoner for taking some strain off you is not where you should be directing your mads.
Basically, later on you should get used to keeping aggro on groups of mobs. Most classes have aoes that they will actively use to make the dungeon go faster. You need to learn to keep aggro on multiple mobs to make everything quicker etc.