The blank canvas is one of infinite possibility. Once you mark it, you have destroyed infinity.
It is very hard to make that first step in writing. When the page is blank, it is in a state of perfection. Once you put down that first word, you increase the chance of imperfection. In fact, the very first word could be one that leads you down the path of a truly lousy story. It is understandable that we would be hesitant to destroy such potential for perfection.
There are a few ways to cope with that. One of them is to start doodling on your page. If you draw airplanes and angry snowmen all around, then fill the gaps with your words, it won't feel like such an atrocity if you end up not caring for those words. Basically, it's hard to desecrate something that has already been desecrated.
Another is to realize that your first draft is not your final draft. No matter how hard you try, you will always need to revise and edit your work. In short, by knowing it is an inevitability that what you write will be imperfect, it takes away the worry. And although that sounds bittersweet, the fact that you also know you can fix all of your mistakes until you do make something wonderful makes up for that.
The greatest thing, though, is realizing that you are creating something greater than a blank page. The page only has infinite potential. When we write on that page, we limit that potential; the next thing we write has to follow from what we have written previously. But no matter what we create, we are bringing something tangible from the abstract world and displaying it for the world to see.
The infinite possibilities of a blank canvas are wonderful and beautiful in their own way. But they are by no means difficult to come buy; you could buy a hundred of them for a dollar. The wonder and beauty you create by destroying that infinity far outweighs the mere potential it promises.