What your girlfriend won't tell you?

It depends on what he has to hide; each individual guy is different and has been through different life experiences.

While there are things a boyfriend in a serious relationship should tell, (e.g. whether he's been married, if he has kids, if he wants to be exclusive with you, etc.) there are some things a guy (or anyone for that matter) might not tell you for different reasons.

If he's cheating, he most likely won't tell you if he feels he's doing nothing wrong, or thinks the rewards outweigh the punishment and wants to keep it going.

If he's had some horrible trauma in his life before he met you, he might not share that because of the feeling of reliving it and it hurting too much.

If he's been in a job that may require him to kill someone or see someone die (e.g. soldier, cop, firefighter, etc.), he may not share that because of other memories it brings and again, it hurting too much to share.

If there's something about you he doesn't necessarily care for, he may not tell that because he feels your positive traits outweigh it and it's not worth hurting your feelings and/or putting a rift in the relationship.

There are many things a boyfriend might not tell you, and it's not always because they mean to be deceptive or mysterious, but because as human being, sometimes we all feel some things are better left unsaid.