If you haven't entered dental school yet, it might be beneficial to take a year or 2 and just work, and see what it's like to look at a computer screen all day every day, doing work you don't care about, knowing that even if you finish your work early, you'll still have to stay there and act busy. Since you're not the boss, the bureaucracy will make decisions you don't like but have to comply with. Promotions and firings are often based more on politics than merit. As a dentist, you can make a difference in people's lives on a daily basis and be a part of your community, you can set the office policies however you want, and the amount of money you make depends on your skills as a dentist and as a businessperson. And you can work as many or as few hours as you want.
Like other posters said, maintaining motivation is easier said than done, but working in an office job for a few years helped me - it motivated me to work really hard in my postbac, and now I'm ready to bust my butt in dental school.