Predictable is a four letter word.
We like the unexpected (in most cases...in surgery or during commercial airline flights, not so much). One of my favorite roller coasters is The Hulk ride at Islands of Adventure. It looks like a regular roller coaster. It begins with the climb, has the descent, the loops. You look at it from the line and think it's just a regular green roller coaster. But it's got a surprise. Something unexpected. It's still a roller coaster. It still scares the crap out of you. It still has all the regular things coasters have, but the touch of the unexpected makes it feel new and exciting, like nothing else that's come before.
Books are the same way. They have to balance a reader's expectations of the genre conventions with doing the unexpected. Follow the conventions too closely and you become predictable. Stray from them, and you upset people who bought your fantasy novel hoping to see the hero triumph at the end.
NICK & NORAH'S INFINITE PLAYLIST is a wonderful contemporary YA romance that gives people exactly what they want in an unconventional way. You know that the two main characters are going to end up together at the end. You know that they'll meet, fall in like, fall out of like, fall into lust, have a fight, make up, and wind up together at the end. You know that before turning the first page. You expect that. And frankly, you'd probably be upset if that didn't happen.
What makes that book so much fun is the unexpected ways in which it happens. Much like the Hulk ride, NICK & NORAH is predictable in an unexpected way.
The truth is that if you're writing a romance, we know the main couple will end up together. If you're writing a mystery, we know your detective will solve the case. If you're writing a dystopian, we know your heroine will outwit the government. Anything else would probably be unsatisfying. But that doesn't mean that the ride should be predictable.