When I cancel time with you so I can spend time doing something else I send the message that I have a hierarchy of friends, and you are definitely not at the top.
To me, being nice means caring about others. It is a high value for me.
There is no such thing as a "promise" in my life. If I say I will be somewhere or spend time with someone then it is the equivalent of a promise. My word is my promise. I will do my absolute best to keep my commitment. And if I cannot keep my word due to emergency, or if I choose to break my word due to personal preference, then my responsibility says I must be up front and immediate with those who I am letting down, and I must accept the consequences of the accountability image I set for myself.
I recently called a friend a "better offer whore" (in a lighthearted tone with all the joking-yet-seriousness I could manage). He had backed out on me for a social event for another social event. When I told him I felt he was taking a better offer than me and that was frustrating because I had spent time and energy on the original plans, he got angry and said he was "making better choices".
I agree he was making better choices - for himself. But making better choices for oneself regardless of how it affects others is not what relational responsibility is about.
Did you consider how your "better choice" would impact others before you made it? If I am not committed to a person or event from the start, okay, but I must let that qualifier be known. If the 'better offer' circumstances are truly extraordinary and I do my best to make it up to or include my friend (or family) in my new plans, then that is great as everyone wins.
Otherwise, living life by the code of "If something better than this comes up then I will back out on you," is not going to cement any trusting long-term relationships. It sends a powerfully negative message about one's values.
When I give my word to anyone about anything I have the highest expectation that I will fulfill my word, and if I don't it should not be handled with just a "Oh by the way I can't." It should be handled promptly, with grace and sensitivity.