Why do girls dump guys over petty things>?

Wanting to have children and start a family is not a "petty thing." It is a major life decision, and if that is an important dream for a women than it is a big deal to her. If a guy doesn't have that dream in common with her it can affet their whole future it they stay together anyways. So it isn't a "petty thing." Anyways, children aren't traps. That's as nice and I'm going to try and explain this to you.