Motive. It's why we do what we do. Everybody has a motive (most of the time). Maybe your motive for cleaning your room is so that you can play on your iPad or on the computer. For most everything, we have a motive. It's important for your character to have one too!
Why is your character doing what they are doing? Why are they plunging into the depths of a magical bookworld or why are they striving so hard to make it to the last day of summer?
Sure, maybe your MC is going into a bookworld because they're curious or don't know what they are getting themselves into, but its important for your character to have a motive at some point.
Maybe they'll jump into a book because they are curious and then they find that in this bookworld that there are dragons taking over this world!
What's their motive then? Maybe they want to get out of that world. Maybe they want to destroy the dragons, so that they can find their sister who accidentally went into the book too?
Get creative with your character's motive! A unique motive can set your book apart from the others!:)
All motives differ depending on what they are doing, but here are some random motives that might just work out with your story!
To make their family or friends proud
To prove that they aren't whatever somebody called them
To find adventure
To retrieve something or someone
To find out what kind of person they are
To complete a goal
To stop somebody from doing something bad
To follow a dream that they have
To help someone
To get revenge
Okay, that's all I have for now. It's a short list, but be creative!
I hope this helped you!:)