Probably the worst part of doing software development is the testing period. You work for ages to create an application and then people get their hands on it and you spend weeks having total strangers tell you that your work isn’t good enough, has bugs, doesn’t have a proper UX etc, etc.
I generally hate the entire process. Some people find good bugs. Some people have insightful things to say about how they use the application. And these are valuable. Especially user feedback on how they want to, or do, use your code.
But those gems are typically hidden in a pool of less than helpful, and sometimes hostile, comments that make the public testing process, for me, to be the least rewarding part of my work.