It seems, in my experience, a lot of people who don't write choose not to. They feel like they "can't", or they don't like what they produce, or they fear others won't like it.
It's oft-repeated that we are our own worst critics, and I certainly see a lot of that. I know people who would like to be writers, or seem so, but they undermine themselves without ever getting under way. They block themselves at every turn, until they just give up in despair and frustration.
I try to be positive and supportive (here's where people who know me in person are gasping but no, really, I do!) I try to show how possible it is. I've even put forth the notion of doing it like one of those "fitness challenge" things that was floating around all of January (you probably saw the Plank one a bajillion times; I know I did): write 100 words the first day, 200 words the second, etc. until you reach the daily total you feel like you want to maintain. Even that was met with wrinkled noses and self doubt.
The thing is: if you don't make yourself write, you won't.
If you don't write, you're not a writer. If you can't give yourself permission to write, you're not a writer.
And, to crib some notes from NaNoWriMo (and probably others), there are stories only you can write, which will never exist.
We are our own worst critics. However, we must also be our own best champions. Doubt is worse when it comes from within. If you don't think something has value, it never will, regardless of what anybody says.
Value yourself. Value your writing.