How do you know you want to be a parent?

As my oldest daughter turns 5 today and my youngest was the craziest I and my husband have ever seen her, this question came to my mind. I did not meet my husband until I was older and finished with all of my school which is a blessing for me, but I always knew I wanted children, but why? My brother had children way before me and I was always asked if I wanted them and I said yes, but not then. I found a career that I could do both, meaning work and raise a family if I was to do it on my own, thankfully that was not the case. So, the question remains, how did I know I wanted to be a parent?
I counsel a lot of couples and I see some with children and some without. There is a difference in the problems, needs and desires of those two groups. I have always believed that the smartest people in the world that decide they do not want children and do not have them. So, I can come up with the reasons not to have them:
1. To save a relationship (doesn’t work)
2. If you have had a rough childhood and decide you do not want children (that is okay!)
3. If you want to focus on your career and feel that is enough for you.
Now, let me try to come up with a list of reasons you should have children? I cannot pin point why I wanted children even the number. We have 2 girls and I get asked all the time if I am going to try for number 3, the answer is no because I know I wanted 2 children. Even when they drive me up the wall, the constant attention and the lack of everything that is me, there is still no better feeling than when one or both of my children say that they love me! I love them with all my heart, just cannot pin point exactly why I wanted to be a parent.
Would love to hear from you about why you do or do not want to be a parent. The high points and the low points on both ends. Let’s hear the stories!