I don't want kids.
Those of you who know me well will not be surprised about this, as I have been saying the same thing since I was about 9 years old (at which point little sister was 1, and I'd come to realise how utterly disinterested in babies I am).
In return for your lack of surprise at my non-maternal revelation, I will not be surprised when you say 'Oh, you'll see, you'll change your mind when your older' or 'I said that at your age too! But my darling(s) are the light of my life' or somesuch twaddle. I've also been hearing that since I was about 9, so I'm not jolted by it. But I am annoyed. It's pretty rude for you to assume that you have a better understanding of my life and what I value in it than I do. It's pretty short-sighted of you to assume that everyone wants the same things you do and will make the same choices.
There are a number of reasons I don't want kids - firstly because I don't like kids very much. They are okay for a couple of hours but the best bit is giving them back to their owners. Secondly, I don't want the responsibility - I am a very selfish person, and I like living my life for me. Thirdly, this planet is fast running out of space, food, and clean air, and contributing to the spiralling population rise is not something I feel comfortable doing.
Admittedly, I may well become hostage to hormones on my 35th Birthday and suddenly decide to become a baby machine. But I know myself well enough to place bets against it, since the idea of pregnancy is revolting - like having a parasitic growth in you for 9 months.
So please, next time someone tells you they don't want to breed, assume that it's a life choice that they have given some thought, and do not dismiss their ideas about their future out of hand.