Cheating, lying, betrayals, abuse, alcoholism, lack of sex, are just a few things that could make a relationship fail. If two people really love each other than chances are, if one of these things were to occur, than they might be able to move forward and in some cases the relationship grows stronger but if something like this happen and something dishonest has happen, than it is better to come clean because it is not fair for one person to give all of their love while the other person lies or keep secrets from them. If something dishonest happens and someone does not tell the truth, than it could come back to you years down the road and if the partner finds out, it will be much more heart breaking for them because they had to wait so long and even though it is old news for the person lying, it is new news for the person finding out this information.
Lying I think for the most part is a big reason why relationships fail and so is cheating because when someone cheats, they will most likely keep it to themselves and they might think as long as they don’t do it again, than they are cleared but it is wrong and the truth might come back to you one day.