Or, you know, don't dream random dreams that don't fit who you are, or really outlandish ones. If I dream to become President of the United States, I'll probably fail (though it looks more achievable than ever after 2016...). However if I want to be a politician, I could definitely do that. If I dream to reach the top of the music charts, I'll probably fail, though I have enough music talent that I could start a music career and not starve to death, I just wouldn't be at the top of any charts. If I want to cure blindness, I'll probably fail, because that's really vague and probably would be accomplished by a team of people. If I get the right education, however, I have the intelligence necessary to graduate with a doctorate degree in the right field to join a research team that would help develop a robotic prosthetic eye, and I would be able to say I contributed to a cure for blindness. I used to have that dream, and I could have achieved it, I was just too lazy. Achieving your dreams is a matter of having the right dream and being willing to bust your ass for it. Really incredible things take absurd effort, not just skill. But that said, you can't just dream anything, it has to be a dream that makes good use of your specific skillset. And yes, some people are hopeless cases with insufficient skills to accomplish anything incredible. Doesn't mean they can't live a happy life though.
Point is, the phrase "follow your dreams" is meant for people with sensible dreams that they're willing to work for, not a blanket statement that anyone can do anything because they just want to hard enough.