Secrets and relationships are often at odds. For instance, you shouldn’t make major purchases on the sly if you and your SO share finances. And it's probably not a good idea to hide the fact that you’re married with children. Those are pretty important facts to disclose. But there are tons of little things it’s OK to hide from your significant other. And if “hiding” sounds bad to you, maybe consider it “keeping the mystery.”
To paraphrase Dan Savage, a relationship is not a deposition. You don’t have to divulge everything about your dating history, personal tastes, or Internet browsing habits. Some things are better left unknown for your own personal comfort, or sometimes theirs.
As long as you’re upfront about what you both want in a relationship and communicate well, then keeping a few things to yourself is fine. Maybe your partner doesn’t need to know exactly how much time you spend playing Candy Crush weekly, or how many secret boards are floating around your Pinterest account. And whatever you wrote in a LiveJournal as a teenager is definitely acceptable off-limits material. Overall, there are a few quirky or personal or embarrassing things that can be left out of your relationship with no harm.