As a mother of three wonderful daughters I have been there and done that. Non eof them came with a manual and there were many times I just wanted to take them back and exchange them for a different model. I was a great parent until I had children! I was trained as a teacher and had worked for years with children. There is nothing like having your own to challenge you. This is the most difficult thing I have ever done and yet the most rewarding.
So why is it so difficult?  The short answer is that we are not trained to be a parent -we just learn the way our parents taught us. This method of teaching is the worst kind. No other occupation allows you to watch some one and then just go and do it with out guidance supervision. The other problem is that our partner also leanred the same way but potentially in an opposite way. Then we have the impact of all of the years of parenting that the two parents bring to the tiny new born. Its a perfect storm for conflict, tension, mistakes and in some cases serious harm. Babies brains start to pick up on the mother's feelings in the womb. Their brain picks up alcohol, drugs, voice, stress hormones, before they come into the world they are already prepared to deal with adversity or safety. Then it gets more difficult from there. Even the most calm present parent has to say no sometimes and even this can have an impact on the child. Imagine a challenging child in a family where there is little tolerance for difficult behavior, noise, conflict. Their brain soon starts to build neurons telling the child I am not lovable, I am a bad person, It is all my fault. The key to parent ing is crucial before birth and after for at least 6 years. To be available to the child when in distress, to meet their needs, to calm them when needed, to listen to them, to set firm limits are  all so essential for a healthy child to develop. You are their brain for many years by helping them to learn to self regulate, solve their own problems, be aware of themselves in relation to others, to take responsibility for their mistakes. As parents you are teaching them all day, everyday how to be the kindest, most  thoughtful, regulated, human being. Everything else is a bonus.