system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why do toddlers love to hand you things?

I'm babysitting for my nephew (20 months old) and for the past hour we've done nothing but essentially play fetch. He's grabbed just about every toy in the room and runs to bring them to me. By the time I've tossed it back, he's grabbed another and is running it over to me with a smile on his face as wide as the Mississippi. 

I'm curious as to what part of development makes this so joyous for him.
gpt
In a recent post on here talking about kids and films/TV, the top answer mentioned that one thing that makes kids so prone to certain kinds of movies (and to watching them over and over again) is that the Toddler brain thrives on predictability.  

Facing a world that can be scary and unpredictable (and having a sort of inborn biological drive to learn about the world through positive and negative feedback) the toddler quickly fixes on anything that has a predictable positive result.  This is especially true -- cribbing from that answer -- when the toddler has the power to do something they already understand (like get a toy) and still get the same result (positive interaction with you.) 

Eventually they'll tire of it or get distracted, but to them that kind of predictability and control is as valuable as hitting GM in overwatch or watching whatever the last episode was of GoT that you thought was good.