I also think resentment is a key issue.
I didn't want children originally but dp did.
Then I fell pregnant on the pill and wanted to keep the baby which dp was delighted with.
All the way through my pregnancy he was a star, and in the early days probably did more than me, even once he was back at work and I wasn't (I had quite a difficult recovery and no fucking idea what I was doing) . He certainly couldn't be accused of not pulling his weight.
Now ds is a bit older and it's a generally even split on the workload (and mental load) we're a very happy three.
Our shared interests haven't dried up, we've kept our own friends as well as made some new 'parents' ones. Our sex life isn't shot because we only have one dc. Again only having one means there are no sibling squabbles so the house is peaceful. Ds is also well behaved which helps because he doesn't stress us out.
From what I can tell though, we seem to be the exception with regards to dps involvement in family life (school, appointments, clubs etc) being greater than normal, which is a bit of a shame I think.
I can easily see how resentment could easily set in if both partners don't pull their weight once a child comes along. If dp hadn't been as hands on as he is or either of us had dc against our better judgement I doubt it would have been anything like as plain sailing as it has been for us.