Why do people impose double standards like this?

People that do things like that are ignorant. Sometimes emergencies come up and plans have to get canceled but if I make plans I will not break them unless it is pretty much an emergency. I hate when people ditch me so I wouldnt do that to someone else. People that do things like that are inconsiderate of other peoples feelings. My sister went out the other night with her friends she had made plans to go out with them a week ago. Then all the sudden this guy wants to hang out with her so she goes to the bar for awhile with her friends then goes to hang out with the guy. Her friend is mad at her for ditching her at the bar. My sister said that her friend wasent alone and that she made plans with two people so she had to do both. I said that she made plans with her friend first so she should have told the guy that she would hang out with him another night. I would be mad if I was in my sisters friends shoes as well. This isnt the first time I've heard of people getting mad at her for ditching them at the bar two other people have said it to be to previously.