scarecrow_of_oz said:
alcohol! It may not be an excuse, but it sure is a reason.
I agree with the cliche that alcohol's a bad excuse, but it's not a reason, either, in my opinion. All alcohol does is lower your inhibitions so you're just more likely to say, think, and act out the things you'd otherwise realise are inappropriate/shouldn't be done/thought/said/whatever
With that, cheating is pretty natural for people. Evolutionarily, if we could get away with it, we're probably better off for it (on the assumption we only cheat with people who have better genes than our current girl/boyfriend)... so people cheat because we're hardwired to do so. Similarly, people don't like cheating because we're hardwired to do so: it's a waste of time and resources if the person you're putting all that 'investment' into doesn't reciprocate and puts their own resources into someone else.
As m_isk said (but i don't know if he meant it in this way): we cheat because we can.