I started off working on my startup as a single founder and found it enjoyable, especially the coding part. What I underestimated was the emotional challenge and that's where having a cofounder helps tremendously. I was fortunate enough to find a cofounder and while the level of stress is still high, its much easier to deal with having somebody in the trenches with you. As far as the later stages of a startup, very few startups even get to that point so I wouldn't worry so much.