You don't have to go to a doctor everytime you have a cold. If you have a simple return and can follow the instructions, doing your own taxes is probably a good idea.
But, even TurboTax, with it's prompts and questions can still be challenging if you have a complex return. You have to understand the questions to answer them correctly. Sometimes you have to know where to look to start the list of questions for the particular type of income you want to report or deduction you are entitled to take. Stock trades, self-employment, real estate, depreciation of property and several other topics require at least a basic understanding of the issues. Even the Secretary of the Treasury, Timothy Geithner, admitted to Congress that he couldn't figure out how to report his foreign income in TurboTax.
Not all software is created equal or as easy to use as another. All tax preparers are not equal either. Like any professional, they all have different training and experience. You need to find the right program or professional for you.