The way I think of eustress:

We all know if we overload ourselves with work, exercise, trauma, etc... It becomes detrimental you our health. On the flip side if we do nothing but sit, eat, and veg all day, this is also detrimental to our health.

Eustress is that aristotelian mean.

Much like our muscles, if you want to grow you have to put them under stress... Not too much where it's over fatigued, but not so little there's no challenge.

Humans require stress in order to grow. It's killing yourself to try and do everything, and it's killing yourself to try and do nothing.

We must have purpose in life that makes the stress worth it. And when you find that balance life becomes amazing!