JacktheRabbit
Explorer
In all seriousness, I tell them to be a Ranger.
Animals in combat is a class feature, just like casting spells or entering rage or inspiring someone. If you want the feature then take the class.
I know it is a bit mean but it keeps things simple. I don't have to worry about how to make beast companions (and maybe familiars) somehow "better" than any animal that any character can purchase. I also don't have to worry about a player of a Ranger feeling upset because someone stole a bit of their limelight simply by buying an animal.
I want to buy a mastiff have it trained to attack on command. NOPE, you want a pet you should have played a ranger.....
So does that mean...
I want to buy some healing pots for my next adventure. NOPE, you want to have healing you should have been a cleric......
And....
I want to hire a local performer to entertain the tavern so everyone is distracted while I rob them. Nope, you want to entertain you should have been a bard....
You get the point. A beastmaster ranger has a pet far superior to any pet another player may buy and train, just like a clerics healing will be superior to the purchase of some potions. Saying no because you dont want the complexity is one thing, it might be lazy, but hey your the DM and obviously player wishes are inferior to your wishes. Trying to say you are saying no because it might outshine the party Ranger is a false argument and very weak.