why he is so gloomy..?

dude Eminem's childhood has been full of hardships and struggle.So when he sings he shares his sorrows wih all his fans with his songs like 'When I am gone', 'Cleaning out my closet', 'Mocking bird'. So no one should expect eminem to sing happy romantic songs. He is great the way he is and he should never change.

hey dude u need to know a lot about eminem's past

If you say he is sad in 1 or 2 dancing numbers...that means that you should probably watch more Eminem videos...or listen more of his songs.TO answer your question.."eminem sings sad" because he is talking about his life and about many other things that didn't made him happy in the past.

well he has had a tough time and that reflects in his muziq and his song writings.. that's it.

Because it is what sells in his part. It is what his audience can relate to. His fans are usually "suburban kids" and this is what relates to them. Or if not, this is what they want to hear. Coz people who are into him and not into hiphop probably listens to emo too. Notice that Em also doesn't write much black-american slang on his songs.