Try to look at it from the bosses point of view, they are looking at the bottom line, having the work covered. Also, did you give your boss enough advance notice that you had an appointment? If you dropped it on them at the last minute maybe he/she got flustered. Bosses are human too.
Don't take it personally, I tell my boss in advance, I have an appt on this day and time, she says ok. But she doesn't want me to leave if I am having a bad day, because my work won't get done. She cares if the work gets done, not that I have bipolar, etc.
I would ask how much advance notice the boss would like for appts, and if there is anything else he/she would like you to do when you have to be away-before you go.