Actually I believe being in denial can be healthy.
This is especially true of someone who may have been born in poverty and everything around them indicates they'll never be "anything". They may tell their friends they're going to be rich or travel the world and everyone laughs at them or tells them they're living in denial. They don't share her/his vision of what is possible.
On the one hand we tell children "anything is possible" or "go after your dreams" but in the (fine print) there is a clause that reads:
"If you haven't accomplished it by such and such date let it go!"
Sometimes in order to get ahead you have to live in denial of present circumstances. So much has been written about creative visualization, affirmations, and positive thinking as methods to jump start the subconscious mind to help us get to where we want to be.
"I am the greatest, I said that even before I knew I was."
- Muhammad Ali
Although I believe there are lots of people who are unrealistic or living in denial based upon their (efforts) or (lack of self discipline) in pursuing what they "claim" they want. Nevertheless I do think being in "denial" is almost a requirement to obtain massive success when everything around you says it's impossible. And yet there are countless "rags to riches" stories.
To beat the odds one has to live in denial of their present circumstances. They can't allow other people to tell them what is possible or post an "expiration date" stamp on their dreams.