I fear you may be fighting a losing battle. It sounds as though you are a good parent with old fashioned values of right and wrong that have been picked up by your daughter.
This is oviously not the case with the parents of her friends who as children of a 'ME' society have instilled this selfish ideal into their children. Her friends cry and mope when they fail to get their own way because as behaviorial tactic it obviously works against their parents.
Children are very quick to learn new stratagies and are always willing to push the envelope of parental tolerance.
Stick to your guns, let her cry and mope, until she learns it doesn't work on you. If you give in she wins you lose it's that simple.
Reward the cheery, kind, loving helpful caring child, it is truly unfortunate that she is in a minority these days but the future of the world needs children like her, not to mention parents like you.