Complex sentence: Limerence can often be what is meant when one has intense feelings of attachment and preoccupations with the love object, and can be experienced as intense joy or as extreme despair, depending on whether the feelings are reciprocated.
Split into simpler sentences: Limerence can often be what is meant when one expresses having intense feelings of attachment and preoccupations with the love object. It can be experienced as intense joy or as extreme despair, depending on whether the feelings are reciprocated.