1. Your sex stories.
Your guy doesn't need to picture you getting freaky with other guys (or girls). If you divulge those stories to him, you'll make him insecure, jealous, and hurt over things that shouldn't really matter anymore. Sure, it's the past and it's been over for a while now, but who said feelings are rational or logical? If you've been in a relationship before, your guy already assumes you've done things. He doesn't need confirmation that you've had sex with so and so.
2. Your hookups.
Why would you tell your guy that you've hooked up with people (and whom you've hooked up with), if he didn't ask? While it's great to be honest, know that these things can be downplayed (which isn't the same as lying). Just tell him it was "some guy from college" and that you guys don't even talk anymore (if it's true). Don't forget to assure him because his heart's bound to be wounded.
3. Your past, in detail.
If your ex was a cheating prick, okay, let your boyfriend know. But you don't have to tell your guy about the happy days you've had with your ex, or anything else that's positive. It'll make him wonder why you're with him, not with your ex, especially when you seem like you're gushing. Don't let him think you're not over your past.
4. That you don't like someone in his family
Because it's not going to do any good in your relationship; he'll feel torn that he has to choose between you and his family, which he both loves. What more, it's not like you guys are married and your relationship/family takes precedence. At this point of dating, it's enough for him to know that his mom or dad frightens you a bit. He doesn't need to know that you think his baby sister is a bitch, his brother an asshole. Just be that good girlfriend and smile everything away.
5. That you don't trust or like his friends.
He can probably tell from your voice or your texts that you're worried about him every time he goes out with his barkada. And that's because you assume he'll do or say crazy things with them, the whole "boys will be boys" thing. That's okay. But if you tell your guy that you think this friend of his is a bad influence, or you think this other guy is a jerk, you better be prepared for him to get mad at you. You don't know his friends as well as he does, and you only see that "bro-y" side of theirs, not the sensitive sides he knows. And because it could be your guy influencing them do certain things, telling him his friends are jerks could mean that you think he's a jerk too. Not good.
6. If guys ask for your number/tried hitting on you.
So you had a girls' night out, and you girls went partying. He doesn't need you to tell him that guys hit on you and asked for your number. He'll mistake it for you trying to make him jealous (which he wouldn't like), or you actually wanting to give your number away but couldn't (which he'd dislike more). It might even lead him to imagine you flirting with other guys, and that'll torture him. If your guy finds you hot (as in, conventionally hot), he knows guys would flock to you. You don't need to remind him of the competition, even if to you there's really none.
7. Your password.
This might be a rare case, where your guy will ask for your password. (Don't we often hear about the girls wanting to know their guy's password?) In any case, none of you should be forced to give your passwords away. That'll just tempt you into checking each other's emails or Facebook messages, and that'll open your relationship up to trouble. You'll see things you don't like, or things you can easily misinterpret because you don't know the context. Worse, you might see him planning a surprise party for you, so good luck acting really overwhelmed when you've spoiled something good.
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