Have you ever noticed and realized everything you do in life sums up to one actual literal thing? Do you know what that one thing is? I think you guessed correct. Breeding. It doesnt make any sense at all but at the same time you would think life means more than just breeding but in reality, nothing is more important than reproducing!
Why did your parents want to have kids? Ask them. One of their answers may be "I always wanted to have kids." "The family needs to keep growing!." "I want legacies of me and my family."
The whole point they are trying to make is saying that they dont want their family to die out and not continue in the world we call earth. So with that said, you must continue breeding, your kids must continue breeding! Everyone is selfish enough to say they want to continue their families legacy going and not have a problem about it.
So when choosing a partner occurs, evaluations are always taken out with more meaning. A guy sees a girl with nice boobs and a pretty face, in his eyes, he is just being horny, but in reality he analyzed her by saying her breast are big meaning they can hold enough milk for my babies. Her face is standard enough to have kids with her and be sure that they would be able to find a mate. Think about it!!!! Its crazy how we as humans are animal like in many regards.