No, I meant stresses other than parenthood. When you have to work 2 jobs, have to decide whether to use your paycheck to pay overdue bills or buy groceries, and never get enough sleep, it's quite difficult to remain rational while your child is acting irrationally. The stresses of parenthood (never enough sleep, worrying if you are doing the right thing) just exacerbates those. After all, we aren't robots. Emotional state is a tough thing to override.
When your kids destroy your sleep pattern and cry most of the time when they are awake during the day, good luck staying rational and yourself. Everyone has limits and kids are very good at finding them.
Kids don't destroy your sleep pattern unless you abuse them by going against millions of years of co-sleeping and throw them in a crib in a different room (doubling their risk of SIDS). In a year and a half we've had exactly one "troubled night" and that's because she had hand foot and mouth disease.
Children cry because something is wrong. Sometimes that may be "I want to pull the cats tail but she's running away" but more often than not it's something that can be addressed by a parent. Love, attention, and affection has solved every bout of crying so far.
I think you were lucky. My sister's kids were all different. One was extremely easy and the other one was very difficult the first few years. Same parents, same methods, different outcome. People are different,so are kids.