Well, as I've gotten older I've noticed more that it pisses people off more than it hurts them. Because of this I just avoid making future plans all together. I think this is the reason why I tend to do things alone because the whole plan thing never really works out well for me. I have a really difficult time doing something I do not want to do because I have obligated myself. And believe me, I think you would rather me just cancel because I am very unpleasant in those types of situations. Now, if this is someone that I am romantically involved with...hold the phone because this is a different scenario! I want to be with you all the time lol.
It also depends on what your asking me to do. If you want me to pick up your kids from daycare, take you to the airport, meet you at the emergency room, help you move, if you're stranded at 3am 3 hours away...things where you really need my help, I will be there 100%. But it if it's stuff like going to the park, shopping, movie dates-if I don't feel like it at the time we are set to go I may not because in my mind you don't need me if that makes any sense.
With other types it doesn't matter if it's a chicken dinner at Applebee's or watching you graduate Summa Cum Laude from Harvard University...a cancellation is a big deal! I can get a good sense of who's who, and the people who get really up in arms over the whole cancelling thing I don't make any plans with. I'll call them up on a whim and ask them if they want to get together. On the other hand, if you are the type that doesn't get crazy about it I will usually follow through with set plans because you're not holding me to anything-I feel at ease.
I know with myself (I don't know about other ISFP's) I don't want to hear about my flightiness because I already know and making me relive it is going to make me defensive. Your best bet is to either let me know it is hurting your feelings (because I don't like people being hurt). I will then either do a better job at keeping my word or do a better job at being honest and saying no (which is another problem I tend to have).
I hope that makes things clearer (probably not lol)!