It is certainly safer to travel with a group, but "dangerous" is too harsh of a word for solo traveling. Of course, you always have to make sure you're safe. However, if you have a little sane judgment, you will be able to protect yourself from most dangers.
Maybe most of this tips you already knew about, or read them before somewhere else.. but I think I have some unique ones. Anyway, I'll add my tips -- or rather, things I sometimes had to learn the hard way. This applies mainly to sketchy countries.