Why do we stress over the smallest things?
This is highly individual, and related to the personality trait neurotocism.
From wiki:


  Individuals who score high on neuroticism are more likely than the
  average to experience such feelings as anxiety, anger, envy, guilt,
  and depressed mood.[2] They respond more poorly to stressors, are more
  likely to interpret ordinary situations as threatening, and minor
  frustrations as hopelessly difficult.
  http://en.wikipedia.org/wiki/Neuroticism