"Many of us have experienced trauma in our life and while it is quite normal to share this with our significant other, the level to which it is shared varies," Nalin says. You may notice signs that they've had a tough time, but it's important for them to talk about it only once they're ready.

"Your partner may choose to keep the trauma to [themselves] entirely or reveal part of it," Nalin says. "The reasons for this are multiple. Some traumas are too painful to revisit and your partner may not have completely faced the situation. In other situations, your partner may fear being judged or misunderstood and may feel safer keeping details to [themselves.]"

You can always let them know you'll be down to listen and help in any way possible, if they ever want to share. But if they can't or don't want to right now, that's fine, too.

While some people may be willing to talk about their fears, or chat about phobias, other folks prefer keeping these things to themselves. "While we typically share our fears with our partners, there are other deep-seated fears that may not be revealed," Nalin says. "These are our most personal fears."

And it's completely understandable why your partner may not want to go there, or say too much. "Sharing them exposes our vulnerabilities in a very deep way and some people are not comfortable with that," Nalin says. "Additionally, we also may not be comfortable facing these fears head on by talking about them out loud."

This is something your partner may want to work on privately in therapy, or something they'll be able to open up about in the future — once they're ready.

"While some individuals are comfortable sharing mental health issues with a supportive partner, others are not," Nalin says, which is why you may never really know the full extent of what's going on inside your partner's head.

"Mental health still holds a stigma and it is not an easy subject to discuss," he says. "For those who suffer from a mental health problem, disclosing this may leave them open to judgement or misunderstanding. This is why your partner may not wish to share it."

They may also not be fully aware of what they're dealing with, and thus not able to talk about it. Since mental health can be a complex and personal thing, don't fault yourself for not knowing everything there is to know.

As Nalin says, family dynamics are complex and can trigger sensitive emotions, so it's not uncommon for true feelings to be kept to private. Every relationship is different, too, in terms of how close you may or may not be to each other's families, and what problems might be brewing.

If you're spending holidays together or going on vacations, your partner's feelings may take on more meaning than if you were more distant. But as long as the dynamic between the group remains generally peaceful, it's fine for them to keep certain feelings and thoughts private.

You can, however, ask how your partner feels if you suspect something's bothering them. They may be relieved to get it off their chest, once they know it's safe to bring up.

While you may talk about your relationship histories, and discuss things like exes, it won't be possible — or necessary — to know everything about your partner's past. And some experts say that can actually be a good thing, especially when it comes to sexual history.

Many people think talking about sex can increase trust between a couple, Jorge Fernandez, LCSW, an individual and family psychotherapist, tells Bustle. But this topic usually has little to do with your current life as a couple, and creates more potential for insecurities as to what may, or may not, have happened, he says.

Again, everyone is different — this is something you two may want to talk about, and it may not bother you in the slightest. But it's also OK if your partner doesn't want to share, or if you don't know every detail.

While it's important to talk about money in your relationship, especially if you live together and have shared bills and other financial goals, you may not know what your partner does with their money throughout the day, Fernandez says.

In fact, it's not necessary to tell each other about every coffee you buy, and depending on the status of your relationship, you may not even talk about money much at all. If you are worried about their spending, though, you can bring it up.

"Try checking in later on how they're doing money-wise this month," Fernandez says. Little check-ins like these can ensure you both remain on the same page, and that you have the same goals for your money.

If you're together, then you'll probably want to talk about your day, share passing thoughts, and have ongoing discussions. But it's also healthy to have your own time and space, and to keep a few things to yourselves. And that can mean not knowing what the other is thinking on a minute-to-minute basis.

In fact, as Fernandez says, "the most successful relationships are ones in which each person gets to keep a sense of themselves as well as who they are as a partner. Be your own person, and want your partner to be their own person."

It's great to create the type of relationship where you and your partner feel comfortable talking about uncomfortable things with each other. But some topics are more difficult to bring up than others, including all things related to career and success.

For example, if your partner is struggling at work, they may not let on about it. "It's hard for anyone to be open as to their shortcomings," Fernandez says. "[Work] is no exception and, perhaps, one of the hardest."

All you can do in this situation is remain as supportive as possible, and let your partner know you're willing to listen if they need to vent, hash out new plans, or simply want to talk about what's going wrong.

"Some people are very private about their spirituality, especially if their views are not considered normal or mainstream," Jonathan Bennett, relationship and dating expert at Double Trust Dating, tells Bustle. So there's a chance your partner is into all sorts of interesting things, he says, and that you don't even know.

It can also be tough to fully grasp what someone's spirituality means to them, which adds yet another layer to the mystery. If it's important to you that you talk more about it, though, don't be afraid to ask your partner. You may find that you have a lot in common in this area, and that talking about it leads to great conversations.

You may not know everything, but "you should know what makes your partner tick," Bennett says. "Connecting to your partner on a deeper level typically goes beyond knowing facts. It involves intimately connecting to your partner’s core values and aspirations." If you know about them, and who they are as a person, you're off to a good start.