Okay, not to sound dramatic or anything. And no, it doesn't have to do with a cheating partner either. I am a mother of a teenage girl who has always been bubbly and popular, but over the past few months I noticed a few changes in her and she just wouldn't let me help her. She wouldn't share anything. As a mother, it is extremely difficult seeing your little-girl-who-shares-everything to grow into this tight-lipped person. So, as a last resort, I used Xnspy and thank god I did. Turns out she was being bullied by a group of students from her school regarding her sudden growth spurt and the apparent physical changes. Things were getting out of hand. If I hadn't intervened timely, my daughter would have done something drastic.
I would surely recommend all parents to have this app on their phone's kids. They won't always tell you the emotional battles they fight while growing up.