Another topic of discussion? Paranoia.
There are relationships I've seen crumble to dust because one or both partners unreasonably assume the worst at all times. I listen, or am forced to listen, to bizarre dances of something that resembles an "investigation" except without the evidence gathering part that end in totally concincing one that the other is cheating. They might even establish that their partner did in fact lie to them about something at some point but still make the jump to "cheated on me" out of nowhere when it is more likely that person just lied about something else. Very sad to see things like this...
The other extreme is the blind eye, where someone has ample evidence, in the form of photographs no less, and refuses to acknowledge any of it in the name of "having faith". After all, what kind of weak faith would I have if it could be destroyed by contrary evidence?
Really a very important thing to help keep a balance is just trying to be logical and gathering information before reaching a conclusion. Reach a conclusion based on the best available information, and that's your best bet. You may be wrong either way, but it's got the highest chance of success and it's always open to revision with future information.
That said, a dash of paranoia, tempured with self control, can keep both people in a relationship in line. Some may not like this view, but hey why do negative feelings ALWAYS have to be a bad thing? Depression, for example, isn't all THAT bad, as it motivates us to get up and DO something. There's a nice evolutionary advantage to all of it, when kept in check.