We have a tendency to dismiss ideas before we've given them a genuine chance to exist outside of our (or someone else's) skull.

Usually this dismissal is a side-effect of the following three things:

1. Preference

2. Insecurity

3. Prejudice (due to past experience)

Let's say you prefer the color red. Somebody throws out an idea that involves the color green. You might dismiss the idea simply because it doesn't involve the color red. This is dismissal due to preference.

Let's say you are insecure about the sound of your voice. Somebody throws out an idea that involves you being recorded. You might dismiss the idea simply because it makes you feel insecure. This is dismissal due to insecurity.

Let's say you are prejudice again flutes. Why? Maybe your ex-girlfriend played the fucking flute. Somebody throws out an idea that involves an intricate flute harmony. You might dismiss the idea simply because it reminds you of your ex-girlfriend. This is dismissal due to past experience.

The bottom line is that dismissing ideas before they've had the chance to take form outside your skull is wholly unproductive. It gums up the creative process. When you or someone else has an idea, try the idea. Get it out of your head and onto the page or the canvas or the demo or the screen.

Then and only then are you allowed to dismiss the idea.