I understand this feeling as well, and from my own experience I have to say that it is usually the behavior / actions of the deceitful partner that stirs the mistrust in us in the first place.

And then there are those that have trust issues that are displaced upon a faithful loved one from the untrustworthy partner - the deceitful partners mind starts to play tricks on them by saying "if I can do this then she/he can too."

I speak from experience from when my husband once mistrusted me due to the fact that he had misbehaved in the past... and he thought "if I could she could" (truth be told, I had just changed my mind that morning and left to walk the dog around the lake when my husband had decided to stop by the house to visit with me while driving to another job - and since I had told him that morning I wouldn't be going any where that day his mind went the only place it could given his own past and he thought I was having an affair).

My husbands fear of this affair only grew stronger in his mind as he drove by the house two other times that day and did not see my car parked in the drive way... the car was parked over to the far right of the yard under a few shady trees for some relief from the hot Florida sun. (lol - go figure)

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HUMANS - COMPLICATED - for SURE!