Ask HN: How do you know what you're working on is worth working on?

How do you know that the idea you want to work on or are working on is worth the time, energy and money? Not talking about side projects that you whipped up over the weekend, but medium term projects that you want to monetise, or even want to market to a medium to large audience (upwards of 100 daily/weekly users)

I see so many posts on reddit, people asking for validation. So I made a community-driven platform for it, I am attaching that link only: https://ideavo.tripivo.co.in

But my question remains, if you don't get any validation, why to take that risk of doing it?