I'm sure this isn't the worst reason in the world but I was quite shocked by this! Years ago I had a boss who told me she only had kids because she was bored of working and her husband wouldn't let her work part time. At the time I knew her her children were 7 and 5 and she readily admitted that she wished she had never had them as they cramped her style! She wasn't a bad mum, I don't think, just a reluctant one.
Dh and I (in common with many people I guess) spent two years deciding whether or not we were ready to start a family - it's such a big decision!