1. You Are Setting Your Relationship Back
After finding out that you went through their phone behind their back, if your partner feels that you violated their trust in you, there might be a huge setback in your relationship. Even if they don’t break up with you, they might close themselves up from you for a long time. It might get very hard for you to bring everything back to how it was. It won’t matter whether you thought they were cheating on you or not, your insecurity will speak for itself. It will make them second-guess their own decision of being in a relationship with you.
After reading all these reasons, if you still have the urge to go through that phone, I think there is a huge question mark dangling over your relationship. Perhaps you should start thinking more about the relationship than that phone you want to snoop in. Either you are not with the right person, or they are not the person you want to be with. Either way you have a bigger issue than that phone in your hand.