You need to create a protagonist the reader cares about to ensure the reader is pulled through your book, always wondering what is going to happen to the main character. Once you have the reader on the hook you have to work to ensure the reader remains engaged with that character and the story the character travels through.
To paraphrase the Lester Dent Plot Formula, the writer has to heap problem after problem onto the main character to interest the reader in what the outcome will be, but also to prove the main character is worthy of their happy (?) ending.
Looking at Elizabeth Bennett from Pride and Prejudice we come to know a character who is steadfast, worthy of a happy ending but who, importantly, has to endure setbacks, social humiliation and the dashing of her hopes before winning the ending we always hoped she would achieve.