Think about all the barriers that we have to overcome every time we pick a new book to read. Is this the kind of book I usually like? Is it too heavy? Too light? Will it bore/scare/offend me? There are many psychological barriers which prevent readers from taking risks with their reading.
Readers tend to drift into comfort zones, always reading the same authors or the same genres and limiting their reading adventure by cutting off whole areas: 'I only read factual books,' 'I never read American books,' 'I hate science fiction.' In reality, of course, these readers would find something to please them if they allowed themselves to explore.
As promoters of reading we need to respect readers and their choices, but also to understand that we can intervene in readers' choices and encourage exploration.