Seems to me a government mandated program might seem more limitless and be less personal and thus you don't feel like your taking advantage of the kindness of caring people or stopping resources going to those who are possibly even more needy.

On a related note: One of the things I somewhat like about taxes for welfare rather than "rely on the largess of charitable billionaires/private entities" argument is that it helps ensure expenses from charitable acts wouldn't be a drain on a business competing. Thus 'asshole' private entities wouldn't have a concrete market advantage over 'nice' private entities.