When I was in grad school I once had a kidney stone so bad that I ended up in an emergency room on Thanksgiving Day. They gave me some fairly strong opiate intravenously. If you've ever had something like this, you understand the appeal of heroin. Especially coming after a week of being curled up into a ball with pain. The feeling when we sold was a less intense but longer lasting version of this.
In retrospect, we could have made Viaweb a lot less painful by spending/raising less money. That was what made it so stressful. But startups were more expensive then. Or at least, everyone thought they had to be.