Goodbye Toddler, Hello Son

A couple of years, err, decades, ago, my mother bought me a coat. I think I was going off to college, or perhaps it was my winter break from my freshman year in college, whenever. As she was paying at the counter, I gave her a hug and kissed her and said "thanks mom!". The checkout lady moaned "awww" and sighed, longing for affection like that from her child. She said to my mom "I wish my son would kiss me every once in a while!" My mother mentioned my brother wouldn't ever kiss her either, and they commiserated as only mothers can.
For some reason, that story stuck with me through all those years. I had never given a second thought as to how many times I hugged, kissed or thanked my mom. I just did it because I was taught to appreciate gifts given to me. I gave hugs and kisses regularly because we were an affectionate family that hugged and kissed each other.
But boys seem to be different when it comes to affection. When my son was born, I kept thinking about that department store discussion my mom had with the checkout clerk, and I spent every moment I could, hugging and kissing my son.
But now my little baby boy has turned 4, and has begun rejecting my hugs and kisses, opting for climbing, jumping, running and hide and seek.
How could this have happened? I've never gone a day without kissing my big guy before. Now I must be content with only a goodnight kiss? And when will that become "yucky"? Why is it my son will happily shove an earthworm into his mouth, but shies away from his mommy kissing him on the cheek? Is it something in toddlerhood that makes them want mommy's hugs and kisses, and the end of toddlerhood means the end of childhood hugs and kisses?
I know I still have a lot of kisses in my future, but the weaning of childhood affection has begun, and mommy's feeling the pain.