The purpose of a glare is to convey to the deep self of the other emotional indifference, the opposite of love, to show danger in place of safety and a capability for cruelty.
A glare is a statement that any emotional bond has been switched off, that the person has the ability to cause the other harm and not care. As such it is a signal of a breaking or broken relationship, for relationships are all about feeling loved and safe.
In your glare is an expansion of a cold emptiness, a complete disregard for my wellbeing that I never knew was in you. I deserve better. I deserve to find real love instead of this poisonous farce.
All things have a time and place. A well placed glare and a foe who deserves the rebuke can avoid a fight or further confrontation, for it conveys the realness of the emotion to the other very with great effectiveness and clarity. Yet a glare to a person you love is a warning sign of emotional coldness creeping in, one that should always be taken seriously.