I love it when people tell me I can't do something. It makes me realize how free I am and it gives me a challenge to conquer. When I say 'free' I mean free from self-imposed limitations others have shackled themselves with.
Last time someone told me I couldn't do something I went through 9 months of blood sweat, tears and endless hours of R&D work. In the end I learned a lot about subjects I hadn't studied in-depth before, improved my troubleshooting skills, and most importantly I proved that I could in fact do that thing I was told "can't be done". There is immense satisfaction in that, and it fuels the desire to do more things that "can't be done".
Men have gone to the moon. They have harnessed the power of the atom. They have decoded the genome. In light of the great achievements of humankind why would a person think they are incapable of building a simple mechanical device with less than a dozen parts? If you want to do something all that needs to be done is to study, acquire the resources, and accomplish the task through determination and work.
-Ben