Sudden grief or any abnormal event like divorce, work issues, relationships with friends and family and financial problems could break down the metal and emotional balance and could invite such a disorder called severe depression.
Even the hormonal changes or imbalance can cause a depression at some extent. But now a days there are thousands of reason that cause low mood, lack of interest, aversion to activities which further affects on your thinking and daily life activities. Finally it leads to depression.
In some cases sudden mood swings could happen due to the seasonable changes. For example in most of the European and foreign countries it’s very common to have a low mood or lack of interest in daily activities. Especially in winter when it socially becomes hard to meet up people and have daily basis conversations. Science also supports the fact that depression seems more common in women than men.