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The mood swings may be as harmless as a general annoyance or severe enough that they are keeping the friend up all night. Take notes in a journal to help you identify patterns. You may also be able to spot other factors that may contribute to the mood swings. Some mental illnesses have mood swings as a symptom. Your roommate may be experiencing an undiagnosed psychiatric disorder. Mood swings can be a symptom or even a result of bipolar disorder, borderline personality disorder, ADHD or even anxiety. Look up the symptoms of each and compare your roommate’s behaviors to see if anything fits. Keep in mind, your goal is not to diagnose your roommate. You are simply trying to learn more about his or her behaviors so that you can have a better understanding. See if they have noticed the behavior. Do this discreetly. You might start by asking other friends if they see the moodiness when you aren’t around. Finding out can help you understand if it happens all the time or only at home. If it's strictly at home, you may want to consider whether the two of you are well-suited if he or she only acts like this in your presence.  If the mood swings are something you have seen occur with other people as well, ask them for suggestions as to how they handled it. "So, you say Terry is moody around you every now and then, too, huh? What do you do when this happens?" Explore how many people in your roommate’s family have similar issues and see if there is a pattern as far as mental health issues running in the family. If the mood swings have been interfering with his or her life, it may be a sign of a larger problem. It can be helpful to remind your roommate of the affect his or her mood swings actually has on life for both of you. Your roommate may not notice the issue on a daily basis, so be prepared.  You might show compassion by saying "I can see that your mood brings you down some days. It also causes tension between us. Have you thought about seeing a counselor or someone you can talk to professionally?" Volunteer to go to the doctor with your roommate for emotional support.
Identify patterns of moodiness. Conduct some research. Talk to your roommate’s friends and family. Suggest that your roommate see a doctor or counselor.