Giving depth to your characters is one of the most important ways of increasing character sympathy.
Make your characters care about things other than the plot. Remember that your character’s life doesn’t begin when the book begins.
If everything your character thinks about and does is focused around the plot then your character will feel fake to your readers.
Beware this mistake when it comes to creating love interests for your main character. Rather than being a person, the love interest simply becomes a role in the plot with little true personality. IE Edward from Twilight or Ramona from “Scott Pilgrim vs the World”
Ask yourself of your characters, “What do they want most in life? Particularly if the plot had never happened to them?”
Give your characters random interests, and a couple of big hopes and dreams that extend beyond the life of the plot.
Some of people’s favorite sections of books revolve around the characters focusing on their passions outside the main plot. IE A scene in the WoT where Perrin simply makes door latches, or an episode in Star Trek where Picard is stuck inside an alien probe where he lives an entire life, has a family, learns to play the flute, and so on.
Note: Your job as a novelist is to learn how to work like a stage magician and make coincidences seem plausible.
Note: There are many different types of relation centered plot dynamics besides a romance: Wise mentor and his student, best friends who start out hating each other, the irritable dragon and his tamer, etc.