You must understand that some people are naturally born pessimists. It means that they see negative sides of things first and will criticize before acknowledging the positives later grudgingly. So, when they moan, know that they're just being themselves.

It will help you much.

Now, these people are like salt in soups; You DEFINITELY need them around you and in your life, but in small quantity.
They consistently help you to understand that it is good to dream and feel excited about things, but in moving forward also, you must prepare for the bad too.

Pessimists see the bad aspect of ideas, businesses and projects before you as a planner can spot them, and their seemingly "negative" expertise to others can be an asset for you in execution. Every large group needs one pessimist at least.

They serve as reality checks.

As much as you need a pessimist, make sure to always filter any criticism they give. Be sure you are certain of executing an idea before ever showing it to them.

Let the pessimism they throw to your idea or goal be a reality check, just to fine-tune your idea, not to stop it.