This question has been surfacing a lot within my social network connections as of late, and it is a question that should be unpacked.
Technology is extremely pervasive within American society, so it is not uncommon to see individuals constantly on their phones throughout the day and night. People use their phones at home, at work, at school, at the gym, at a bar, out to eat, while walking, and sadly while driving too. The attachment we have to our phones is often viewed as unhealthy in the eyes of most psychological studies, and to some degree in our eyes too.
The panic we feel when we misplace or lose our phone is virtually on the verge of a traumatic experience. Anxiety often rises, and disorientation ensues. But this does not only happen when we misplace or lose our phone, this also sometimes happens when someone is looking at our phone without our permission, and this includes our significant other.
Everyone's phone is tailored and suited to one individual, themselves, so when you or your significant other looks at each other's phone without permission, it is often considered an invasion of privacy. The defense that is often raised is "Why can I not see your phone?" or "Why can I not have your passcode?"
Now it is possible that they may be hiding something, but it is also possible that they may just not want to share their phone. You may state "How can I trust you if you will not let me see your phone?" But the underlying question is do you trust she/he if you are asking that?
The simple answer is NO. If you trust your significant other, there would essentially be no need to see their phone. You should trust that your significant other is holding up to their end in this relationship, and then if/when they would like to share with you, they will.
People often share more, when they do not feel forced. Your significant other and yourself must leave room and space for autonomy and privacy. No one knows everything, and no one needs to know everything. Is it not better when you and your significant other voluntarily share information, instead of feeling like you have to because of some sort of relationship pressure?
Why should a phone come between the two of you? And honestly, a phone is often the subject, but this could apply to many other things such as a laptop, a journal, or a memento.
Now, there is usually at least one more follow up statement that arises in these situations, and it is often, "I do not trust my significant other" or "My significant other says that she/he does not trust me." And if that is the case and it is often repeated, then it is probably best that the two of you not move forward in your relationship.
Without trust, there is nothing. If your significant other does not trust you or you do not trust them, then how can you have a healthy relationship? No one is physically with their significant other at all times, so when they are alone or you are alone, how can it be known that something nefarious is not going on? Why would you subject yourself to that level of anxiety and disorientation? But most importantly, how do you know your significant other's true feelings and vice versa?
Trust is not a commodity, it is a necessity in relationships. And if there is trust, why would you need to see their phone?