I think most writers will say that at the start of each book they think, 'I'm not sure I can do this.' But eventually, you reach a magical point where the story suddenly becomes real to you, and you become totally invested in it.
Why do you keep reading a book? Usually to find out what happens. Why do you give up and stop reading it? There may be lots of reasons. But often the answer is you don't care what happens. So what makes the difference between caring and not caring? The author's cruelty. And the reader's sympathy...it takes a mean author to write a good story.
Smart writers really take their time in investing in backstories and characters. As a viewer, you have to invest in them and love them before you can chip away at what's going on more on a deeper level with secondary characters.
People buy box sets, and they sit for a whole weekend with a computer on their lap in bed, and they watch two seasons back-to-back of a show. They are invested in the person within that arc or the dynamics of those people - the relationships - and it doesn't matter to them if they're watching it on an iPhone or a cinema screen.
Back in the day as a kid, I was really drawn to the Hulk because it just felt so human and was probably one of the first stories that I felt emotionally invested in and not just thought it was really cool. You really feel for that person and put yourself in that situation.
It's all about the integrity of their characters. They [Marvel] care so much about the loyalty and integrity of each and every character and all of their stories. They trust and love their readership. They're the ones who have invested in these stories.
When the reader hears strong echoes of his or her own life and beliefs, he or she is apt to become more invested in the story.
People want to find out what happens to the characters, and want to keep reading, and turning the pages.