That IS a very good question, so I ******d it and this is what I found from one site. I find this quite interesting info and I imagine many of us can identify with many of these symptoms.

Dopamine deficiency signs/symptoms:
Physically fatigued easily
Sleep too much and trouble getting out of bed
Reduced ability to feel pleasure
Flat, bored, apathetic
Low drive, motivation & enthusiasm
Depressed
Difficulty getting through a task even when interesting to me
Procrastinator/little urgency
Shy/introvert
Mentally fatigued easily
Difficulty paying attention and concentrating
Slow thinker and/or slow to learn new ideas
Put on weight easily
Crave uppers (e.g. caffeine/sugar/nicotine/diet soft drinks/cocaine/amphetamines)
Use these improve energy/motivation/mood
Prone to addictions (e.g. alcohol)/addictive personality
Light headedness
Reduced libido and/or impotence
Family history of depression/alcoholism/ADD
Dopamine levels may be low due to a combination of genetic and acquired reasons. Dopamine can be raised effectively using either nutrient based therapies or medications. Dopamine is synthesized form the amino acid tyrosine.

Factors which reduce dopamine levels:
Chronic stress
Inadequate sleep
Hypothyroidism
Lead, arsenic and cadmium exposure
Under-methylation
Tyrosine (precursor) deficiency
Magnesium, iron, zinc & vitamins B3/B6/C/D deficiency
Excess copper levels
Genetic dopamine receptor abnormalities
Chronic opioid, alcohol & marijuana use
Adrenal insufficiency
Glutathione deficiency
Parkinson's Disease
Influenza
Estrogen deficiency
Human growth hormone deficiency