In the realm of adventure sports and outdoor pursuits going solo can be viewed as an unnecessary risk and poor choice, especially if an accident occurs. There are immeasurable circumstances and variables that go into a person's decisions, and this also includes when and how accidents occur.
In its simplest sense, risk is the chance of a consequence occurring. Those consequences are generally negative and harmful. However, in taking risks there is the possibility of rewards to be gained. Risk is a human construct for us to try to make sense of a complicated, dangerous, and chaotic world. It's a way for us to facilitate our decisions and explain the outcomes of past circumstances. Some of these are based off of careful study and analysis, while at other times, these are based off of anecdotal experiences.
There is a commonly held notion that doing something alone is inherently more dangerous and risky than doing it with others. I don't believe that is a completely accurate or helpful assertion. From personal experience, and speaking with others, I tend to make more conservative decisions and choices when I am by myself. Because I know that there is no one there to help me 'pick up the pieces' outside of myself.
When out with others, I've found that I will push myself, especially with experienced others, because I believe that they will be there to help me. I have also been out with others where the notions of staying together as a group to support each other 'fell apart.' Without clear expectations, and people holding themselves to them, the benefits of being in a group diminish.
Whether you go solo or with a group there are still risks; so just because others are with you does not decrease your chances of something bad happening. What it does provide is additional people to help. However, the experiences and decisions of those people can sometimes make the situation worse. The perspective on the decision to go solo, or with others, becomes more circumstantial. A person's skill level and fitness, the environmental conditions, the remoteness and difficulty of the location, and other factors attribute to the choice being made.