If you say, "I'm a doctor" or "I'm a college professor," you have instant credibility. It doesn't matter how good you actually are at your job, or how long you've been doing it, or even if you are happy. You have a respectable title. But stating "I am a writer" brings a lot more challenges. There isn't an instant association that people can make. Their thoughts may drift to, "Is she a starving artist? Does she sit around in her pjs all day? How long can she keep this up until she has to get a real job?" (or so I worry).
I know not everyone I meet has these thoughts, especially others in creative fields. But there is a stigma attached, and although I know I shouldn't care, this stigma helps motivate me to work hard, even if it's just to shut up my self-doubt and the perhaps false assumptions I have about other peoples' reactions to my career of choice.
But the scariest part about stating my profession is that when people here "writer," many seem to expect that you can list books you've published or major magazines that you write for. It fails to impress them that you have rewritten someone's website, or have a blog, or even write for a local newspaper. And yes, I realize that what I do for a living doesn't need to impress anyone, but at the same time, I can't help but want respect.
I am a writer, and darn proud of it. But I want other people in my life to be proud of it too, and that motivates me to put fingers to keyboard every day.