I'm sure there are things you want to say to people in your life, but don't. Even if you're the kind of person who talks a lot, or is very open and honest most of the time, we all have those things we keep inside because we're afraid of letting them out. Some of them we shouldn't say, because they would accomplish nothing but hurting others. Some of them we really should, because our relationships would greatly improve.
Most people have a tendency to want to avoid conflicts in their life. If it's possible to let things slide for the sake of keeping peace, sometimes it's easier. But in the world of fiction (and when you think about it, real life, too), conflicts advance the plot. Sometimes these conflicts come from external forces. Sometimes, they just come from feelings. The ones from the outside are obvious immediately, and more common in action/adventure/fantasy books. The ones from the inside are often in the books just set in the ordinary world, where we still need plenty of intrigue to keep things moving. In these books, those quiet conflicts need to come into the open sooner.
So, how to we bridge this gap between reality and fiction?
Let unsaid things be conflicts too. Knowing someone is keeping something from you, or you have to keep a secret from someone you love, can deepen the emotion in your book in realistic ways. Make sure you have a good reason for keeping things unsaid. "You lied to me?" can get annoying if there's no good reason for the lie.
Let things come out in smaller, subtler ways. We can say things we don't mean when we're angry, and afraid of telling someone the real reason why. Let your characters have these emotional moments, where they hint at the deeper issues without outright saying it.
Bring the big conflict at the right moment. You don't have to wait until the end of the book, just let things unfold naturally. Maybe some time after an angry outburst, the characters in conflict should sit down for a heart to heart. Maybe things are resolved there, maybe they only begin working on the issues. Think about your characters backstory and the history of the relationship. If your going to "resolve" the conflict in the middle of the book, maybe don't let it disappear entirely. Instead, maybe it's something your character is more aware of and working on throughout the rest of the story. There's no perfect formula for this. The "right moment" will be different for every character and conflict situation. Some characters are going to be more forgiving than others. You just have to play around with it until it feels right.