Relationship is about trust and been vulnerable, both parties are expected to be open to themselves and not keep secrets from each other.
You will worry that he will find out
If there’s something serious in your past or something about your character that you’re hiding from him, it will constantly keep you up at nights. Chances are high that the truth will come to light someday and you don’t want him to find out in the wrong way. This can negatively impact your relationship.
It’s not as bad as you think
When you keep secrets from your partner, they tend to seem bigger in your head than they really are. Sometimes, talking about it can give you perspective and you may even realize that things are not as earth-shattering as they seemed in your head.
It makes you closer
Sharing something imperfect about you can make you and your partner feel closer to each other. When you find someone that accepts this ugly part of you, you create a bond that is stronger than most. You feel safer and more secure with him.