Cheating in a relationship is always wrong, destructive and leaves scars.
Cheater may end up cheating because he/she isn’t feeling well in the relationship but doesn’t have the backbone to do something constructive about it and resorts to running away (emotionally) and cheats.
Or the reason for cheating could be a moment of opportunity that is taken advantage of (“S/he was just there, at work/school and we just hit it off”). Or, as in many cases, it’s something you do drunk: the drinking lowers the inhibitions and suddenly there’s just someone there that you accidentally slip into (hate when that happens!).