“Gain loss theory”

The gain-loss principle states that as one person's
opinion of someone else becomes more favorable,
that other person will tend to develop a more
favorable opinion of the first person. This increase
is more marked in people with an initially less
favorable opinion of the other person.

a theory of interpersonal attraction stating that
people's like (or dislike) for another person is
more strongly affected by the degree to which they
believe they have gone up (or down) in that
individual's estimation than by the unvarying degree
to which they think they are attractive (or unattractive)
to that person. ...

☝
これは心理学用語だからちょっと難しいが、
日本の学生たちは、「ギャップ萌え」と言うそうだ。