You find out he is a thinker.
(Even if he doesn't want to show it.)
He throws himself into the things he loves. He's sincere that way.
You humbly realize you are the only mother he will ever have, and so pity him a little (!) and try to be patient when he, say, cries for 10 minutes over not being able to decide which shoes to wear.
You want to show him the world and open doors for him so he will reach out and grab hold of life and living.
Some days, you're not sure if the end of the day will ever come because you can't bear squaring off with him. And if he yells, "You're not my friend! I don't like you!" and you say, "Oh good, that means I'm doing my job," and then he panics a little and says, "No, I want you and me to be together forever," then you think you just might be doing something right after all with this little life that you're guiding.
And while you wonder what kind of person he will become, you achingly wish for him not to grow up too soon. And even though he's five years old today, he wasn't too big to ask you to lie next to him while he fell asleep (which hasn't happened in two years).
You know what he likes and doesn't like and can usually predict how he'll react in a certain situation. You learn the best way to get him to take a bath. Or try something new.
You learn a lot about a person in five years, including realizing there's still a lot to be discovered about him. And when that person is your son, mostly you learn that you couldn't live without him.