Designing is a way of expressing emotions, processing thoughts, opinions or ideas that we get from our surroundings. It's what everyone does, whether they know it or not, because we are creative beings.
I've always loved to know how things work and why certain things are like this, but not like that - to understand. A big part of the use of this information or understanding is to create something new with it, to take it into a different context where it could become a whole new being. It's a genius way of "stealing", or maybe more like borrowing, from all over and creating your own invention.
As a child I dreamed of becoming an inventor, like they used to be in comics or cartoons, but then realized that there is no such job as "inventor". Only years later, after starting to study engineering and design, did I realize that my inner child would have that dream come true after all.
I design because I like to see improvement. It pains me to see broken things remain broken, or just something being stuck and making no progress. Even when listening to someone talk about their issues or struggles they're having, my reflex is to think about a solution for them, even though (usually) they really don't want to hear solutions. Solving problems and knowing that I've made a difference gives me a feeling of achieving. So, you probably could say that I design out of a selfish need to feel accomplished and achieved.