I suspect that one of the factors at play here is how different people get ideas, and how much external stimulus they need, in order to do so.
For example, some of my friends can't bear their own company, and have very few interests which don't involve other people, or activities, to stimulate them and make them feel alive - but other friends enjoy much more solitary pursuits and yet are never bored, or short of ideas.
Each group often hold totally different views as to what is needed to promote and enhance creativity.
I myself think that there's a role both for solitude and the input of other people in most creative activities, but at different stages, depending on the type of activity involved.
For example, I love listening in to other people's conversations and observing how they react, when I'm not working on a story - as that then informs my writing, and helps to make it more believable.
However, when it comes to the time to sit down and actually do the writing itself, I then need total silence and to be left alone, so that I can "hear" what I'm writing, in my head.
It can take ages to get into the writing "zone" when I'm by myself, so having other people around tends to make it impossible.
I also find the same thing happens when working on visual art projects, when I prefer to work largely alone at first, while developing an idea, but then to see what other people's opinions are, once I have some idea of where I'm hoping to go with it.
At that stage, other people's input can lead me to completely change direction, but - at other times, and equally usefully - sometimes their responses will make me even more sure that I want to go ahead with what I'm already doing, even if they don't think I should.
In this sort of situation, it's as if the challenge posed by other people's ideas and opinions forces you to be much clearer about how much you believe in what you're doing, and then to fully commit to it. Once that happens, it seems much easier to find the energy needed to see your project through to completion.
Another interesting point which is linked to this, and to what others have said, is to what degree collaboration benefits creative outcomes - and on what type of collaboration it is: Competitive, or truly collaborative.
Personality must also be a factor, as - while some people become more creative when under pressure, whether as a result of deadlines or because they are in competition with other people - others can find that these things kill creativity, and render them completely unable to produce the quality of work they're otherwise capable of.
Some people find that conflict makes them more creative, too - while others find it so stressful that it makes them wholly unable to think creatively.
I'm also quite curious as to whether being left- or right-brained has any effect on whether people need solitude or company in order to be at their most creative.