Making a connection: sympathizing with the characters

Recently, I was asked a question for my upcoming blog tour for the release of If I Speak True in January. It was: What do you think makes a well-written story? I kind of surprised myself when I didn't even have to think before giving my answer: well-written characters, tension, mystery, and humor.

Later, as I was thinking about my interview answers, I had to laugh. Apparently, a plot--of any kind--wasn't even on my list of necessities. I've found that I love to be in the characters' heads more than anything, which leads me to today's topic: How do you keep that happy medium of delving into your characters' heads enough that you readers connect with them, but not so much that they can't stand reading it?

I know some people have felt that Pity Isn't An Option is way too into Jonas and Hattie's heads, and I get it. Sometimes readers are in the mood for a story like that, and sometimes not. Sometimes they're in the mood to get to know characters and coast along on their journeys, other times they'd rather see a wide scheme of things played out before they reach the back cover.

So here's my question for you: How, as writers, do you attain this? I've often seen reviews where readers share their frustrations of feeling no connection to the main characters (or the opposite--they were tired of the characters and wanted more action), and I've wondered, Does the author's voice have anything to do with this? Is it a conflict of personalities (the reader's and the MCs) that makes this such a touchy experience? Or, Did the author simply miss the opportunity to get you to sympathize with the character/s by not helping you understand where they're emotionally coming from, first?

Enabling the reader to sympathize with the characters is, after all, part of writing a "well-written" book in the first place--something all of us writers are striving for. Below, I've listed a few books that really made me connect, sympathize and root for the characters. 

How about you? We'd love to hear about your own personal character connections in the comments!