Quoted for truth....
With me it was that I couldn't trust what she was doing when I wasn't around or what she was doing when she was out with "friends". Which I ended up being right about. When we first got married we had sex 3 times a day at least. I'd wake up and she'd be on my dick. I'd come home from lunch, I'd end up getting no food. For almost 2 years of our dating and marriage was like this.
I think most situations like this there's something else bothering the relationship. Whether it's stress, depression, financial problems, etc.