Motivation is how you convey why a character is doing what they’re doing. I go into great detail about it in my upcoming book, WRITING IRRESISTIBLE KIDLIT, which comes out in late October/early November. But it all boils down to this, and if you want to write this on a Post It for your computer monitor, that might not be a bad idea:
I don’t care about what a character is doing until I know why they’re doing it.
All reader investment and emotion comes from caring. All character emotion comes from the events of the plot and how they rub up against their motivations, objectives, wants, and needs. If you don’t put any thought into the latter elements–or if you don’t work to convey those to the reader–nobody will care or feel anything.
And feeling is the biggest thing you want to inspire in your audience.