This method can exercise my imagination and creative thinking. I really like this way. 

Separation anxiety is derived from Sigmund Freud and John Bowlby's attachment theory. According to Bowlby, there are individual differences in the way attachment develops, with four main patterns: secure attachment, anxious-avoidant attachment, disorganized attachment, and anxious- ambivalent attachment. Of these, anxious-ambivalent attachment is the most relevant to SAD (Bretherton 1992). 
It is common in infants and young children, but research suggests that separation anxiety is more common in adults (Manicavasagar 1998). In most cases, separation anxiety is triggered by stressful life events, especially the loss of a loved one or pet, but it may also be caused by parental divorce, changes in school or community, natural disasters, or forced separation of the individual from the person they attach to. There is also evidence that a common factor in separation anxiety may be an overreaction to negative social signals (facial expressions) (Stein 2002). 

Cortisol is a steroid hormone that helps the body respond to stress. It’s sometimes called the “stress hormone.” That’s because levels of cortisol in the body spike during times of high stress. Most obvious when Anxiety.
When this hormone level is significantly too high, it reduces the amount of serotonin that provides a sense of calm and happiness.
Where is it from?
The pituitary gland and hypothalamus of your brain regions work together to direct the adrenal glands to produce more or less cortisol, in essence acting as the control mechanism regarding how much cortisol is made.
Your adrenal glands -- triangle-shaped organs at the top of your kidneys -- make cortisol. From the adrenal glands, cortisol can be released directly into the bloodstream. 

Craig, J., Baron-Cohen, S., and Scott, F. (2001). Drawing ability in autism: a window into the imagination. Isr. J. Psychiatry Relat. Sci. 38, 242–253.

Crone, D. M., O’Connell, E. E., Tyson, P. J., Clark-Stone, F., Opher, S., and James, D. V. (2013). ‘Art Lift’ intervention to improve mental well-being: an observational study from U.K. general practice. Int. J. Ment. Health Nurs. 22, 279–286. doi: 10.1111/j.1447-0349.2012.00862.x

Futuress. (2021). Against Performative Positivity. [online] Available at: https://futuress.org/magazine/against-performative-positivity/.

optimistminds.com. (2020). Puppy Separation Anxiety (A Brief Guide) | OptimistMinds. [online] Available at: https://optimistminds.com/puppy-separation-anxiety/

optimistminds.com. (2020). Separation Anxiety From Boyfriend (9 Tips To Cope) | OptimistMinds. [online] Available at: https://optimistminds.com/separation-anxiety-from-boyfriend/.

Sexy, G. are (2018). Fighting Mental Illnesses and Disorders One Illustration at a Time. [online] Geeks are Sexy Technology News.