I have come to realize through my own mistakes as well as relationships that one major cause of distrust is contributed merely because that person has been hiding as well. I would say 80 percent of relationships i was either involved with personally or from people i was close to the person who does the accusing is probably the one who should be accused. The other major reason is hurt from a previous MEANINGFUL relationship. If you were involved in a relationship where the other half is your entire world and that person broke your heart, any intimate relationship after that may take a long time to trust entirely. As much as we all probably say we WON'T compare our previous ex's, whether consciously or unconsciously, we tend to anyway.