This question had haunted me for quite some time, sometimes, it surfaces when I am feeling hopeless. The argument had some truth to it, we did not choose to come into this world, we just are. But some just don’t want to admit it, they will give some motivational, psychobabble, but it is not a good counterargument to the fact that we really did not choose to come into this world.
You could deny the argument by giving motivational speeches, but it will further angers the asker as it is not true. And dealing with antsy teenagers who asked a legitimate question, and got an unsatisfactory answer, especially with the wrong premise, will make you respect your authority less, and rebel more.
After all, to reach old age with positive attitudes, I would assume you would have to have this delusion in “life is good”, else, you would have killed yourself long ago. Your situation would blind you to give a neutral point of view. I do agree that sometimes, to ask this question, you have to have depression already.
This guy, Ed Lake gave a good philosophical perspective to this question in this aeon.co website.
Parents who have offsprings who ask them this could give a better answer instead, or point them to this. Or risk them hating you forever.