Life is like that sometimes.
When people cancel at the last minute, it throws everything into chaos. You don't know what to do for the time being.
When they try to schedule a later date, it interferes with plans you had for that date and potentially any following dates depending on the nature of what was cancelled (i.e, a surgery or a dentist appointment where you get braces could throw things out of wack for quite a while)
All we can do to get through these sorts of experiences is to try and keep going, and maybe, just maybe, it will work out.
This might seem counter intuitive to some people, but if you cancel on me last minute and then offer to have the appointment another time, also last minute, both of those things are about equally stressfull to me.
You just disrupted my plans twice.
Just leave me alone.