Now I know what you are going to say. Your ancestor did not leave behind a diary or journal. That's Ok, mine didn't either. But guess what, someone who was a neighbor, an associate or some collateral relative might have left some writing behind that describe their life and their community.
One way to find out is by using the National Union Catalog of Manuscript Collections (NUCMC), http://www.loc.gov/coll/nucmc/. This Library of Congress manuscript collection catalog provides a one-stop source for searching collections throughout the United States. Now, it does not cover every possible collection in the United States, but it's a great place to start. You can check out what institutions participate by clicking on the link for List of Participating Repositories on the left hand side column of the homepage.
To search NUCMC, Click on the link Searching on OCLC WorldCat located in the middle column of the homepage. You can then conduct a search using a keyword. I would suggest searching on the locality your ancestor lived in. Then you can see what manuscripts are available for that location.
You might be able to locate diaries and journals by searching the collections of a state historical society, local museums and other regional archives. You may also want to try university libraries in the area that your ancestor lived.
Other people's diaries and journals can provide valuable insight to your ancestor's life and the community they were a member of. Yes, this is a source that does not have a convenient index, but it can be a true genealogical gem.