I actually think you *should* be disappointed. You wanted to hear a shiur on a specific topic, but you didn’t. It should be the same disappointment as you would feel if you were told that the topic was changed because the scheduled maggid shiur came down with laryngitis – not an upset disappointment, but a disappointment that you didn’t hear the shiur you wanted to.
Had you gone to the local pizza shop and been told that they ran out of your favorite topping, I would agree that although the normal human emotion would be to feel disappointment, the ideal would be not to. Since, however, we’re discussing talmud Torah, and one is supposed to learn what interests him, disappointment is fully justified.