… because you're sad ...
Could be a number of things..
You could be suffering from depression, not just depressed but the illness depression. If you can't shake this feeling you should talk to your doctor there is medication that will help.
There could be a number of reasons for feeling bad. In my experience in life I have found that the bible can make a major contribution on our attitude about life.
The Bible is an ancient book of wisdom, it provides answers to life’s important questions and can help you to feel better and have a sense of well-being.