William James and Carl Lange proposed in the 1880s that every emotion has a unique pattern of physiological responses (Cannon, 1927), but when researchers studied emotional responses, they found overlapping responses, such as increased heart rate for anger, fear, and surprise. So Schachter and Singer (1962) proposed that all emotions have the same physiological arousal, and what differentiates them is labeling based on cognitive cues. For example, my heart is racing and I look around and see that I am in a street and a car is approaching me; therefore, I must be afraid.
But while physiological responses often overlap among emotions, they are not always the same. For example, blood rushes to the face when angry but away from the face when frightened, and heart rate slows when sad. And in most cases, the cue comes before the arousal and indeed causes the arousal, e.g., my heart is racing because I saw the approaching car. Hence, researchers thought it was impossible to measure emotions, including love.