Belief is a powerful force in our lives. Religion is based on it. Our own personal lives are fueled by it or fouled by a lack of it. If someone believes in you and what you are doing, especially if they are close to you, then that's a strong positive reinforcement to what you are attempting to do. It makes you feel like you're heading down the right path, like nothing can stop you. Lack of belief, however, especially on those days when you're feeling down, or you're having a hard time seeing the light at the end of the tunnel, can be the opposite...it makes you think that those around you either don't think you can accomplish your goals, or that you're serious about your goals, or they simply just don't give a shit.
Now, the latter reaction can also be a sort of positive reinforcement...it can make you say,"Fuck you, I'm going to do this with or with out you, whether you believe in me or not...and I'll be laughing at you for not believing in me when I accomplish my goals."
Don't let naysayers deter you from whatever it is you're trying to get done.