It is not always about when your partner is never stopping to reach your phone and go through your private stuff.
While you might say you trust him or her or vice versa, it is not advisable that your partner checks your phone.
You partner might be facing psychological problems that make him or her to pick your phone from wherever and start 'studying' it.
The following are some psychological problems your partner might be suffering from;
1. He or she is unable to set healthy boundaries
Sometimes it is not anything close to doubts in relationships as many people perceive when a partner grabs your phone, in some cases, the partner cannot stand to boundaries you set.
2. Your partner is not getting enough attention
If there is little communication thus no attention for your partner, they might end up doing what they literally know is not right, and they even don't like doing it, including checking your phone.
3. Your partner is not ready for a serious relationship
When your lover is serious with your relationship, will by default initiate a talk with you when he/she feels something is not right, but if the partner checks your phone when you are away, it shows the level of maturity is low and is not for serious dating.
4. The partner doesn't have trust for himself or herself
It is not about your trust when your lover checks your phone, sometimes it is due to trust issues within themselves.