paying off business/equipment loans, making the practice grow, managing the employees and dealing with sick leave, maternity leave, insurance, benefits, bank accounts, the list goes on. All an associate dentist does is show up to work and do the dentistry. The owner/dentist has to juggle all the managing stuff, small business owner stuff, plus all the dental/medical stuff. It's a lot to deal with on a daily basis.
Then, as was mentioned, you have patients that daily tell you how much they hate you, and you yourself are trying to be perfect with every case, which is impossible when dealing with the human body, it's a very high pressure job.