You know how some people, if someone doubts them, they get fired up and shake a metaphorical fist, yelling I'll show them? This mentality can be a powerful tool. While the motivation to make it won't come from your core, it still pushes you to make something of yourself. And something is better than nothing.
So say, for example, that your friends ("friends") or you parents or other relatives tell you if you're opening up about your dream to run your own company that it'll be too difficult, that you don't have what it takes, that your niche won't have a market, that you just won't make it? Of course you get a counter reaction. You want to show them what you're made of. You want to prove them wrong and get the last laugh. And chances are those people will never make it with their own dreams - that when they tell you you won't make it, they're actually just revealing their fear that you'll become better than them. These people are wind in your sails, and it's up to you if you want to let them go from your life or keep them around to motivate you with their doubtful words.
But then there's another set of people that can doubt you. And here's where it gets dangerous. If a person that's too close to you emotionally - a partner or a best friend - doubts your venture, you might actually listen to them, agree with them, and never even try your idea. You value the opinion of those people too much to ignore them. Their doubt won't be wind in your sails. Their doubt is an anchor, stopping you from steering the ship to your desired direction.
So be very careful who you share your plans with. Your idea might be gold regardless of what your partner of best friend thinks of it. You should listen to your gut. If your heart is calling out to that venture, if you feel passionate about it, it shouldn't matter if someone else thinks it's not a good idea. You should give it a shot.
Conclusion: you don't have to share your ideas with anyone. Sometimes it's best to keep them to yourself. Then you're free to find out in practice if your idea works - and if it can help you create the life of your dreams.