I have observed that when guys become adults they start thinking about girls. Why is this so?
When girls became adults, how do they act?
"There are three things which are too wonderful for me, yes, four which I do not understand: The way of an eagle in the air, the way of a serpent on a rock, the way of a ship in the midst of the sea, and the way of a man with a virgin" (Proverbs 30:18-19).
It won't be accurate to say that boys don't think about girls when they are children. Boys obviously think about their mothers, sisters, and friends with whom they play. What changes is the nature of how they think.
A part of it is biological. Children have no real concept of sex unless someone forces it on them. They are aware that girls are different from boys, but it is on the same order as noting that dog is different from a cat. But after puberty, the awareness of sex intrudes on a person's consciousness. In both sexes, it isn't something that can be overlooked or ignored. And each time sexual thoughts arise, the feedback from the body causes people to pursue it further.
The nature of the awareness is fundamentally different between boys and girls. Boys are more visually attuned, so seeing things or remembering things they have seen typically sets off the mechanism of arousal. Girls are more relationally attuned. What is said and the body language that accompanies it can be triggers for sexual excitement.
A part of what drives boys and girls together is also societal. Even from childhood it is typical for children to pretend to be married and talk about what they will do when they get married. We want to imitate those around us -- especially our parents. Awareness of sex bring another dimension to this expectation to get married.
I'm sure curiosity plays a small part. With new feelings and new capabilities people naturally are curious as to how it all works together and what it is like.
But what ultimately drives it all is a knowledge that you want someone as a personal and intimate companion. "Live joyfully with the wife whom you love all the days of your vain life which He has given you under the sun, all your days of vanity; for that is your portion in life, and in the labor which you perform under the sun" (Ecclesiastes 9:9). Even with family and friends around you, as you get older you realize that there is a hole that needs filling that only a spouse can provide.