Your friend has a great business idea. He's been thinking about it for awhile and he has put some things in place to make it happen. He is excited about it.
He is meeting you for dinner and decides to share his idea with you. The idea is precious and he doesn't want to share it with just anyone. He has known you for a long time and trusts you. He values your thoughts, and is looking forward to hearing what you think. He also wants your support.
You get together. You chat about different things, and then the perfect time comes to talk about The Great Idea.
He shares it with you. As he is talking, you remember something similar from your experience. You feel compelled to tell him about it. The moment he pauses, you burst in. He will know you were really listening, that you 'get' what he is talking about. You say 'Hey! You should go and check out [so and so] on [x] street. They are doing exactly what you are talking about. It's great.'
Suddenly, the energy in the conversation has changed. It drains away. Your friend stops talking and looks tired. Conversation dwindles. You say your good nights, after a few more words, and head out. It's time to go home.
What happened there?
Your brain linked his idea to something you already knew. It's natural; as human beings, we do it all the time. We are constantly looking to link new information, to see the patterns.
However, the last thing your friend needed to hear about his new, unique, brilliant business idea was that someone else had already thought of it and was doing it, successfully or not. There is power in the bubble of belief with a idea, and the power needs to be there to propel the person forward. Your friend does need to know what else is out there in his market niche, but you don't need to be sure he knows it the first time he talks to you about it. And you don't even know for sure that business is in the same niche. In fact, you weren't really listening to him because you were busy linking what he was saying to something else. You also don't know what makes your friend's idea remarkable compared to what else is out there.
So what could you do differently? Notice when those thoughts come into your head. Resist the urge to blurt them out. Practice listening to him instead (I put some thoughts here about that). Be curious. Ask lots of questions. Maybe your questions will help him identify some things he needs to think about. He will leave the conversation feeling heard, and grateful that he had the good judgement to share something important with you.