It's a function of complexity
I have been full circle. I started out doing my taxes when I had only earned income and it all fit on one tab of an Excel sheet. (in fact it was Lotus 123). If you have the time, and your situation is simple, build a spreadsheet that looks like a 1040 with the same line numbers, and you will learn a lot.
Then over the years I got married, got a house, got a rental house, got foreign taxes, AMT, ....I added tabs upon tabs to that spreadsheet. Finally it got so complex I called in a professional preparer. He indeed found things I had not, and for some years I used his company for the peace of mind. It was expensive but worth it.
Then I realized my situation had stabilized, no more new forms every year, every year the return structure was teh same, just different numbers; and decided to try Turbotax. Even with all of the above complexity, the version I needed was only about $90, where the professional fee was more than 10 times that. Guess what? Turbotax worked perfectly even with all that complexity. And guess what? It found stuff that my preparer had not - because it asked questions he had not asked.
So now I am back to doing it myself, but this time with Turbotax. And saving a bundle.
So bottom line: if your situation is relatively straightforward, you can do it yourself, it's not hard. As it gets more complex, I would recommend a preparer until you understand it completely. Once you do understand it, you can use software yourself, even if it is a complex situation. I would not have had the confidence to go direct to software, there is nothing like being able to ask a human a question.
Caveat: You are signing the return, whoever or whatever does the calculations. So take the time to make sure you are comfortable with the answer, however you get there.