Great observation honghanhvt1182 ! I don't think it means you have to be a loner, but I think it can go both ways. That is, I think "loners" may oftentimes become writers. You are correct though, about the research and time it takes to "contemplate on ideas", that certainly lends itself to time alone. Although in many cultures, French and Amsterdammers come to mind, a lot of dialogue between writers happens in social situations, such as coffee houses and cafes.
For myself, I never felt like I fit in when I was younger. I spent a lot of time making art, reading and writing. When I first started writing I didn't think of myself as a "writer", as I'm sure most people don't. But now I do, and when I write, it is almost always alone. Although currently my kids are making a lot of racket watching cartoons and banging toys on the ground so unfortunately I write when I can and can only dream of that kind of solitude haha! Truth be told though, I edited some poetry yesterday as I sat around a bonfire in my backyard, delicious and sweet time alone! Now I consider myself a somewhat accomplished artist and author (I've published one inspirational self-help type book on Amazon and I'm working on putting out a collection of poetry).