“What is treading on eggshells”
(I’m trying to define it)

Maybe it’s:
When a question is perceived as a judgement?

This is the point in a relationship where, you first sense, that you’re views or actions are not entirely welcome;
The moment where mutual trust evaporates, and you realise that acting in someone’s best interests, has become an intrusion; not necessarily because you overstepped the mark; but because you were perceived to do so, and that this is the moment where joyful innocence, morphs into trepidation and uncertainty.

From this point on, you’ll probably be spending a lot of time, being hyper vigilant of your own words and actions; so not to be perceived as intrusive or offensive.

In conclusion, what you’ve lost!…is your freedom!
and what they’ve lost!…is your honesty and innocence.