The passion and unconditional love in a romantic relationship change over time. There is a lot of chemistry involved between two individuals to lead a healthy relationship. Studies show that many neurochemicals are supporting you to fall in love and to maintain that relationship. However, romantic love can be classified down into three distinct categories, such as Lust, Attraction, and Attachment[1][2][3]. Among these categories, Lust and Attraction are relevant in the early stages, whereas Attachment is more associated with mature relationships. These three categories of falling in love are characterized by the release of the set of feel-good chemicals stemming from our brain, which results in specific physical and emotional reactions.

Lust

In the first category of love, an individual is influenced by the release of a bundle of sex hormones and neurotransmitters, which drives one to engage in a sexual encounter with another person.[4]

• In the early stage of love, even a virtual or real presence of the opposite sex is enough to release the neurotransmitter Dopamine (pleasure hormone). The Dopamine produced in the Hypothalamus in response to sexual stimulation further enhances the sexual desire by stimulating the release of sex hormones.
• The sex hormones Testosterone and Estrogen generated in response to the signals from the Hypothalamus also play an essential role in enhancing the sexual craving for both men and women. In addition, the release of Estrogen helps to assist in guiding and coordinating the body by reacting to each brain signals.

Therefore, the first category of love is supplemented by the pleasure and sex hormones. They play significant roles in the stimulation, mood, and emotions of individuals, which results in feelings of excitement and happiness [1][2][3].

Attraction

Lust may not lead to a real romantic relationship. There are other neurochemicals and hormones, which help an individual to succeed in the Attraction. This hormone flood will influence the individual to focus more on the relationship through his/her emotions.

• The presence of Central Nervous System stimulant, Phenylethylamine in the human brain, leads to the release of Norepinephrine and Dopamine. The release of a large amount of Phenylethylamine occurs in the process of falling in love by supplementing more physical and emotional energy, and alertness [5].  
• Additionally, the release of Norepinephrine stimulates the production of Adrenaline hormones and exerts significant impacts on the brain. The levels of these chemicals are responsible for the behavioral changes associated with the increased heartbeat, and restlessness [6].
• The release of a mixture of hormones such as Dopamine (pleasure), Adrenaline (fight or flight) and Norepinephrine (alertness), in the brain makes one behave like a love addict, similar to drug or alcohol addictions [7].
• At this stage, the further decline of Serotonin levels will lead to Obsessive-Compulsive Disorder (OCD) in individuals. That means the decrease in Serotonin influences the person to become obsessive, extremely emotional, and love becomes uncontrollable. During this stage, an increase in the release of stress hormone Cortisol, which allows the body to survive with the emotional conflict [8][9].

The neurochemical influx into the brain will lead to many emotional responses, such as obsession, addiction, nervousness, and anxiety.

Attachment

The third category of love, Attachment, is crucial for long-term relationships. The first two categories of love are exclusively for sexual and romantic relationships. But the third stage of love, Attachment, facilitates friendship, pair bonding, parent-infant bonding, social interactions between individuals, and many other intimacies.

• In this category of love, Dopamine neurotransmitter begins to decline slowly, and the influence of the second category, Attraction, reduces. In the category of Attachment, Dopamine and Norepinephrine are replaced with other neurochemicals such as Oxytocin (binding hormone) and Vasopressin (commitment hormone). These hormones help one to stay connected once the individual gets committed with their partner [10][11].
• The Hypothalamus gland produces Oxytocin hormone, which allows you to maintain and foster healthy interpersonal relationships, healthy psychological boundaries with other people, and long-lasting commitment for entire life. The release of large quantities of Oxytocin hormone occurs during sex, breastfeeding, and childbirth [12].
• In addition, the Hypothalamus gland releases Vasopressin hormone directly into the brain, which also influences the long-term relationships, sexual motivation, and social behavior. Both Oxytocin and Vasopressin play a significant role in leading a monogamous relationship with the partner [13].
• During this stage, the individual will overcome the extreme nervousness due to the decrease in the release of Dopamine and relax more in the relationship by connecting and attaching more with the partner [14].

In the Attachment stage, obsession, and addiction in the partner ends, and the role of new hormones released from the brain is to promote attachment and comfort. Additionally, the hormones released in this stage also help one to calm their anxiety and reduce their stress by enjoying a long-lasting committed relationship.

Conclusion

From a scientific perspective, love is the effort of the brain to trigger and suppress the release of brain chemicals throughout the three categories of a romantic relationship. The neurochemicals involved in all the three categories of love play vital roles in creating and sustaining a meaningful connection between two individuals for committing a long-term relationship. In a relationship, individuals will go through all the three categories of love with the help of their brain chemicals. Depending on the hormonal stimulation, one or another category will be prominent and so you will show varying behaviors in your love relationship.

References

[1] Helen E Fisher et al., Archives of Sexual Behavior, 31, 2002, 413-419.

[2] Helen E Fisher, Human Nature, 9, 1998, 23-52.

[3] Helen E Fisher, Journal of Sex Education and Therapy, 25, 2000, 96-104.

[4] Carl A Ridley et al., The Journal of Sex Research, 43, 2006, 144-153.

[5] Gabriel Miranda Nava, Scholarly Journal of Psychology and Behavioral Sciences, 1, 2018, 48-49.  

[6] https://en.wikipedia.org/wiki/Norepinephrine

[7] Alton M, Neurological Science Journal, 1, 2017, 1-2.

[8] Neal R Swerdlow, International Review of Psychiatry, 7, 1995, 115-129.

[9] Müller-Vahl et al., Scientific Reports, 9, 2019, 1-10.

[10] Navneet Magon et al., Indian Journal of Endocrinology and Metabolism, 15, 2011, 156-161.

[11] Wei-Hsi Chen et al., Advances in Sexual Medicine, 7, 2017, 139-152.

[12] Anna Buchheim et al., Psychoneuroendocrinology, 34, 2009, 1417-1422.

[13] Markus Heinrichs et al., Frontiers in Neuroendocrinology, 30, 2009, 548-557.

[14] Strathearn L, Journal of Neuroendocrinology, 23, 2011, 1054-1068.