A few years ago I met a guy that I liked a lot, but when I told my friends about him, they seemed to "talk me out of" the relationship. I had just broken off an awful engagement, and while I thought I'd waited long enough to date again, my friends convinced me I should wait a while longer.
In looking back, this was terrible advice. My friends were all single and looking, and we had a great time going out together each weekend. When I started dating the new guy, I didn't go out with them as often and they didn't like it. At the time, though, they convinced me to break up with my new boyfriend. They weren't suggesting this to hurt me, rather, they were thinking about only themselves and not me without even realizing it.
Negative friends are just one reason to sometimes keep a relationship secret until you're totally comfortable with it. Here are some others.