A (more or less): I wanted something (encrypted snapshotted online backups); spent a while thinking about how to do it and convinced myself that I could do it; posted to my blog asking if anyone else wanted and would pay for it (about 10% of the people who read that blog entry wrote to say that they would); and then got started writing code.

As for cofounders, I don't have any -- having decided that I knew how to do this, I didn't want to waste time explaining to anyone else how to do this.