Shifts in the mood are normal, but in some people the mood shifts are more dramatic, which happen more frequently, or last longer. In most cases, the individual is suffering from a mood disorder.

The difference between whats normal and normal is quite difficult to elaborate but in Abnormal Psychology if a person fails to execute establishing good relationships to others or fails to go to work or to school then there really is a problem. Same with what you have quoted above, if the sudden changes of moods shifts from depression or mania or even is very dramatic in nature then the person might be suffering from mood disorders. Psychological disorders only kicks in if it is already disrupting our academic,professional and personal life.