What's wrong with short stories?

After looking at various reviews of my free short stories, "Blink" and "Last Dance of a Black Widow" one of the most common complaints I found is... "I expected more" or "I was hoping for more" or "This would have made a good book but it was too short."  Or something similar.  People simply wanted more.  They enjoyed the story but needed to have more of it.  The killer is, these people then give me three out of five stars, not because they didn't enjoy what they read, but because they felt cheated.  Now, readers can't feel cheated monetarily because these stories are free, though I guess they can feel cheated time-wise because they spent ten minutes reading something that they thought never really ended... but that's a different issue altogether.