Anyone who's watched an episode of House knows the value of constructively poking holes in an idea. When you have an idea for a project, find someone who's willing to argue against it early on to refine it.
As writer Todd Henry explains, the role of the devil's advocate exists for a reason: by choosing an opposing viewpoint and asking how they might respond, we find the holes in our logic. It challenges us to improve our idea, rather than accept the first notion that comes to our head. It's not about pessimism but refinement:
It is challenging to argue against yourself. Once you've determined a course of action, it's much easier to jump into execution mode and figure it out as you go. However, in the effort to "ship fast" we often fail to lay a sturdy foundation for our work, and the results can be disastrous. Shipping early shouldn't be equivalent to thoughtlessness. You need to ensure that you are respecting the work by giving it your best mental effort.
Having a devil's advocate against your idea may involve asking another person for input, or simply inhabiting the mind of your critics yourself. A task many of us with reasonable self-doubt are keen to do anyway. Don't let criticisms get you down, but use them to make your idea better before you execute your plan.