Ignorance and bliss:

The study of ignorance is agnoiology.  "Agnosia" is an idea in philosophy and a medical condition.  As a condition, a subject who seems to have normal perception will look at or listen to something but will be clueless that it is actually there. The eye or ear responds but the brain takes no note.

Years ago I ran across a hip but obscure definition of "Agnosia" in the abstract.

"The unknowableness of the object answering to the unknowingness of the subject".  

Some things are indeterminate or mysterious because the limitations of the observor make them so.    Resonating with this is the idea that any measurement by an observor alters the state of the observed. With agnosia, it's as if the reality of the object is affected by the reality of the observor.

Donald Rumsfeld gave a modern twist to this referring to  "Known knowns, known unknowns, unknown unknowns".  The fourth corner of this tetrad is "unknown knowns" - things we know but don't know that we do.

The converse seems sometimes true.
In the charming song, "How much is that doggy in the window, the one with the waggly tail?" the quality of the observed (the waggly tail) is actually a reflection of the affectionate character of the observor.  The dog wouldn't have the defining quality of a waggly tail if the observed dog didn't like the subject.

Falling in love seems a special case of knowing.  Of all the people we half notice, one becomes an overwhelmingly interesting object to know more about.  With this attitude, you suddenly DO learn a lot more about the object of your desire.  A newly met couple will sit talking for hours about every tiny interesting thing in their life.  This despite having been content to be ignorant for years before and even determined to keep the other person uninformed about yourself.

From an old collection of Punch cartoons.