I started writing whilst in primary school, and wrote through most of my classes during high school. I was part of a group of 15 or so friends, and I wrote during lunch times. I wrote during theatre rehearsals. I wrote when friends had homework to do, and when we got older, when they had uni or course work.
I write at work, I write with my partner around and I write with my partner.
Writing alone doesn't always get me writing fast. I get distracted by researching on the net or getting a notification for something. It can be necessary though for difficult scenes.
Overall I find I write best at work. The environment is ideal - as long as I've done all my work and I'm not getting distracted by having to do something, of course!
(Though work does always come first at work, of course. I'm very lucky I work in a place that basically knows what I'm doing and allows me to get away with it, knowing that I won't let anything or anyone down for it.)
I've always told girlfriends in the past that I like writing so leave me alone sometimes and let me do it. I mentioned the other day to my wife I wanted a desk so I could have a nice place to write. When I woke up in the morning she had been searching online for desks.
So having a supportive partner is always great for me. I think maybe for some people it may be easier to write if they are alone, but I find the vast majority of people have partners of some sort that are supportive of them and/or writers themselves.