Why Do People Have Mood Swings OR Seasonal Moods?

I have always been wondering why poeple have mood swings? It seems like a never ending mystery. I hate when people have sudden mood change. I understand there can be unavoiding moments where one may not have control over the situation and can get into depression mode, however, problem arises when it starts becoming a frequent affair. One must realise the affect it can have on others. Its like you are suddenly cutting the link with everyone around you, just because you are in a bad mood or because you have felt bad about something. The worst part is you will dare not discuss the reasons to justify your "so called depressed behaviour" (or rather attitude). How can one not realase how depressing this kind of attitude become for others????