They always say that you shouldn't meet your idols. It is a common saying because people are commonly disappointed when they do.

Needless to say, unless your idol is a literal mythological figure, they are humans too. This means they have flaws, weaknesses, and personal problems like we all do. So, it would be best if you treated them like so. Do you become disappointed when you meet a person you didn't know previously? Maybe you become annoyed or weirded out but you rarely become disappointed. Why is that? Because you didn't have expectations.

But wait you do kind of know the person, that's partially why he/she's your idol. See, you don't. You have the perception that you do, sometimes he/she wants you to think that, but you don't. 

Expectations are part of human nature. Even if you don't know the person and don't even have that perception, you always have expectations. That's how your brain works. It's healthy because it's a way for you to prepare for a situation, although it can increase anxiety, it lets you prepare in advance for a potential danger. Think about it: you see a person at a bar/event/etc. Your brain starts preparing for an eventual encounter, so you interpret that feeling and form an expectation. Based on that, you might decide you want or don't want that situation. So, you will almost always have expectations unless it happens by surprise. How can you avoid disappointment?

You should treat every interaction as an opportunity to learn. If you enter that state of mind not only will you become less disappointed, but you will have better interactions and enjoy them. Even if the encounter doesn't go as you expected it, you will look at it as a learning experience and almost always take something from it.