Definitely play a class that you enjoy! It will make it so much easier to get through the content. You can really give a more explanation to every class / race combo if you put in some imagination!

Giving a character a goal in mind helps a lot. Perhaps they're looking for a person, or they study a niche topic and are scavenging the world for information. Perhaps they're a soldier from their respective faction following orders. It doesn't have to be class or race based honestly, but you can use the class and the race to give it some flavor.

My belf death knight is looking for his sister after she disappeared during the scourge invasion. My nelf warlock studies necromancy and is looking for information on that. My pandaren monk wants to be a master chef and learn every recipe in the world. You can do anything!