LUKE: No, I believe you. If you say there’s no reason for the mood, then there’s no reason for the mood. You’re simply nuts.
LORELAI: Or bipolar. That’s very big nowadays.
Bipolar disorder, once known as manic depression, is a mood disorder where the sufferer goes through periods of severe depression followed by manic periods that might mean feeling abnormally energetic, happy, or irritable. During the manic periods, erratic and impulsive behaviour is often present, as well as an inflated sense of welf worth, and speaking excessively and rapidly.
Although Lorelai probably isn’t bipolar (she’s never diagnosed, doesn’t take medication, and never becomes suicidal or self-harming on screen), it’s interesting that she does seem to have quite a few of the symptoms to some extent. Her mood swings may be due to all the sugar she consumes.
When Lorelai says being bipolar is “very big nowadays” she’s probably referring to those celebrities, such as actress Patty Duke, who were open about their diagnosis, raising awareness and encouraging others to seek help or treatment.