Digging through a friend's personal information on their phone without permission is damaging because of the trust you have broken. If I stole money from you but then offered it back, would you trust me the same now? I gave you the money back but trust is very hard to restore. Same thing with the phone. If I were to look through your personal info on your phone, would you trust me anymore? There are boundaries for everyone- friends, family, strangers. Boundaries are important to life because it offers you freedom and privacy. If we were to have no boundaries, everything would be free to the public and nothing would be private or sacred if you will. So going through a friends phone without permission can be seriously damaging to their trust in you and therefore, will damage your bond.