In his case, it was because I talked to guys who liked me and my bf was very "how can you not see that they are flirting with you?". So I stopped talking to these guys to avoid conflicts and gain my bf's trust, but he could not trust me easily.
In my case, I didn't trust him because of a thing with his ex. She called him a lot, and he talked to her and they sometimes saw each other (only friendly), but it caused me an internal conflict, like "He still wants to see her, why?" We talked about this issue many times until they apparently stopped talking, or so he told me. Either way, that episode was a big blow in the relationship, but I overcame it... apparently.
This morning I was in a very bad mood, sex had gone bad and I was feeling weird overall. He was trying to be gentle and affectionate, but I was feeling odd. Then when he went to shower I did something I apologized for. I had never ever gone through his phone messages before, this was the first time. I really don't know why I did it, but I did.
He had been talking to her via whatsapp, though according to him they had stopped communicating since Dec of 2012, but last message was dated 21st Mar, 2014. The messages weren't flirty at all, and they weren't daily, perhaps 3 times a month. What surprised me is that she was not the one looking for him, it was him initiating most of these chats only to say hi or something.
The last chat went like
He: "hey, how are you?"
Exfg: "Don't contact me again, please. I moved on.
He: "How much did you move on?"
Exgf: Take care, bye.
I talked to him about it, he said he was just trying to be friends and he didn't tell me about it because he knew I'd panic. I told him why then, would she say that. He said she probably saw no point in a friendship anymore.
I believe we love each other, but can't see why this happens.