Also, I have found that I am more motivated to model something if I'm modeling it for someone else and not myself. Recently, I was "hired" by a friend to make a simple radiation logo for his new company. I would never in my life want to make a radiation symbol just to make it. However, when I was asked to make the logo, I changed into a different mode. Suddenly, I felt the need to make it the most perfect radiation symbol of all time. I had an obligation to please my friend and do a good job, so I put a lot of effort into it.

I don't know how old you are or if you're already in the 3D industry or not, but maybe you could look for simple jobs that require a modeler to make something or if you're younger, round up a few friends and ask them what to create. Start your own company and make a logo or something.

Another thing that drives me is the need to accumulate enough models and animations to fill up my portfolio. A good portfolio is needed if you plan to go into the 3D business. Ask yourself "what would an employer like to see in a portfolio?" then make it.