Why do people have children they can't afford?

I know how this question sounds, but I genuinely mean it with no judgment. I have been reading the “We Are the 99%” tumblr (http://wearethe99percent.tumblr.com/), and there is also a situation in my personal life that made me wonder about this. Having kids makes any bad situation (unemployment, low wage job, car problems) much worse, and I am wondering how it transpires that so many people have kids that they have no resources to care for. Does it come about largely because of unforeseeable catastrophes, incorrect expectations about the cost of raising children, accidental babies and a strong pro-life position, something else I’m not considering?

Again, I really do not mean to suggest that anyone in this situation is lazy or stupid or a bad person, and I very strongly believe anyone in this position is entitled to as much public assistance as they need. But I am wondering why it is that this situation seems to come about, and why it happens so often.