The standard "I'm not getting paid for doing this and so should not care what my users feel" mode of thinking is not a good one for an open-source developer.
Despite the fact that they don't pay you - you should pay attention to their comments and answer their questions kindly. They are your best asset.
Treating users like they are your co-developers will turn them into co-developers.
Nevertheless, the core developer will have to do a very large portion of the work himself, if he wishes his project to thrive. ("There is no such thing as a free lunch").