Your questions about depression are an excellent example of cause/effect expository topics. You might use this same format to investigate other mental illness topics such as anxiety, OCD, bipolar disorder, or any other psychological condition. In answering this question, you will need to consider both the increase in awareness which can cause more people to seek help and a diagnosis, along with different stresses and cultural conditions which might be causing an increase. You will need some facts and statistics to prove there is an increase if you want to talk about rising trends. Finally, including personal experience or examples from the media can make this a compelling and interesting paper topic.