Why is it annoying that a ball passes the goal line?

Obviously we, as goalkeepers, don’t like it when a ball crosses the goal line…….

In general: those balls are a reason to get distracted ;-)

We have many reasons to let go of this distraction & I would even advocate that you better leave your goal and D in some cases.
Just imagine: you’re practicing to learn/improve/engrain a specific skill or movement.
Your focus should best be on the awareness of what you are doing. Feeling, sensing, experiencing, evaluating.

But: inside the goalie’s head there’s this little demon that hates balls passing the goal line. So when this ‘reptile-like’ instinct J notices a ball coming towards us goalies, it might try to get some part of our brains to initiate a save…..
And this is likely to interfere with your intention to develop a skill and doing things the right way. Let alone being aware of what your body is doing! Since the urge came up to prevent the ball from crossing the line, etc.

So: a wise thing would be to leave your goal every now and then if you want to focus on the execution of a skill instead of being distracted by a result-eagerness.

Bonus: it will also prevent you from getting a false judgement upon your own action. A save doesn’t mean that you did well (in practice this is of course!!) since you were aiming for a good execution of a skill/movement.

Good practices & remember: doing nothing sets you back. So best follow some kind of plan or scheme during the period when there’s no training, practice etc to stay on top of goalie-things!