Maybe it's the idea of trust within a relationship that leads the guilty party to think that their partner wouldn't go snooping about without being dragged into the mud themselves...I mean cheating is one thing but if someone has invaded another persons privacy that hardly puts them in a good light either. It gives the guilty person a bargaining chip; 'Ok so I broke your trust but you broke mine so there.'

Either that or the person cheating isn't really that bothered by being found out - i mean if they're cheating anyway they're clearly not happy and bound to end the relationship eventually.

These stand out as the most obvious reasons to me but there could be any number of them really; revenge, laziness, forgetfulness...

But one thing's for sure; if someone's cheating and their partner has to go snooping around to find out, both parties are hardly in the clear and need to examine whether such a lack of trust is a good basis for continuing a relationship.