Actually, having high serotonin reminds me more of being a kid than having high a high amount of THC in me. But I do admit, before trying any serotogenic drugs, I had the same view as you: "Smoking makes everything seem so much more interesting/cooler than they really are, almost like when I was a kid."
But you should see what it's like when you're high on something like MDMA.
And I'm pretty sure that the amount of serotonin/sensitivity of the receptors is higher in kids than in adults. Think about it. Serotonin influences memory, how we learn things, emotions. Memory is based on an emotion. Why is it that every time I smell a perfume, or a nice flower/place/whatever that reminds me of my childhood, I immediately have a mini-flashback of that period from childhood, suddenly remembering stuff
This also happens when I listen to a song that dropped when I was peaking off MDMA, the same exact mini-flashback, and remembering stuff, the same feeling.
Why do you think this is? Childhood is the period of our lives when we just need to learn learn learn, so we can deal with life in our adulthood years. So it's important that we learn a lot more during childhood than adulthood, that is a fact. And since serotonin is involved in learning, it is probably one of the neurotransmitters that is present in a bigger amount in children's brains.