Loneliness is a feeling when an object of one's love is absent.
Loneliness is the emotion branching out from sadness's appraisal.
Loneliness is felt when we are alone or we don't have the object of love with us. We feel loneliness when we realize that the object of our altruistic behavior is not here. Loneliness can be called the opposite of happiness.
Crying, the stagnation of various activities, or enhanced empathy for example are displays of loneliness, which is the same as sadness.
The facial expression and behavior of loneliness has the purpose of requesting relief from others. This promotes obtaining a new object of love. Loneliness makes people change their behavior pattern by reappraising and reflecting on their lifestyle, and avoid loneliness which is unpleasant. This is the same as sadness.
Pets can drive away loneliness because pets become the object of love. That some women have an unnaturally large number of potted plants for example is sometimes due to loneliness.