There can be a number of reasons why a person would cheat. It may be that they are not happy in their current relationship, whether they realise it or not.
Cheating can often be a cowardly act in that the person who does it may not want to be with the person they are dating any more, but they do not want to admit it to them, or even to themselves.
Cheating can happen in the heat of the moment, and "cheaters" often use the excuse that they had too much to drink, and did not realise what they were doing.
Of course, being drunk can affect a person's judgement, and we can do things that perhaps we would not usually. I would argue that you do, or at least should, still have a sense of right and wrong, and that if you make the decision to cheat when drunk, it is indicative that something is wrong in your own relationship.