That's really interesting about German law. I didn't know that, so I appreciate learning.

I suspect that if we were having this debate over dinner, we would agree on far more than we disagree on. I think we're speaking past each other because we're operating with slightly different notions of what mentoring implies.

From my perspective, I think that there's an obvious and reasonable expectation that you make best efforts to be a good team player. That means doing your part to participate in planning, knowledge transfer, group morale and of course making yourself available to work through tough problems with people regardless of their skill level.

What I think is far more dubious is the relatively recent slippery slope towards the notion that someone should reasonably be expected that career progress dovetails with some sort of natural law that says you are not being a good human if you aren't willing-to-excited to spend 70% of your productive time in a semi-permanent state of continuous partial attention because the people around you demand priority access to your time and attention, above any of your personal priorities or job responsibilities, often without compensation beyond a rote "thanks".

If you can't ship inside of a deadline because your ephemeral "mentoring" took implicit priority over your actual job, then something is very wrong with people's expectations of how key talent's time should be allocated.

having this talk over dinner sounds nice :-)

german law or courts operate more on common sense than the letter of the law. to be able to reject a task from your boss the task has to be unreasonable. training/mentoring is not unreasonable unless the work is dangerous and/or a trainer needs specific qualifications. running a class that teaches some generic topic like a programming language would probably be unreasonable because you could just hire an external trainer for that. the training has to be rather specific to the individual experience of the senior or some company internal knowledge to be reasonable. so this doesn't apply to the scenarios you seem to have in mind. in any case from what i read it is recommended to talk to a lawyer before refusing.

but also, if the request interferes with your ability to do your normal work then you can and should speak up and you can reasonably refuse if your boss doesn't make accommodations for that. if he doesn't then refusal is reasonable, not because training wouldn't be your job, but because your workload has increased, which is not ok.