[azangru]

> What is so hard that people find estimating their work?

Unpredictability for any new work. Even knowing my codebase well, I sometimes forget how changing some piece of code will require changes in other places. And without having done exactly the same work before, I cannot know how difficult it will be and how long it will take.

Dependency on other teammates or other teams. If another team is building something that your work depends on, you have no way of knowing when you'll be finished. If the code you write needs to be reviewed by other team members, and then you'll have to incorporate the suggested changes, this makes it harder for you to say how long something will take.