In what way are voluntary response samples often biased?
Voluntary response samples are often biased because people want to answer the question.
A person may volunteer a response because they have an interest in the topic or because they have a particularly strong opinion (negative or positive).
For example, if you're conducting a survey on people's opinion on childhood vaccinations, you might have people responding who have very strong opinions one way or the other. People who have moderate opinions may decide they don't care enough about the topic to take the time to answer.
You also may find that people who are interested in a topic will respond. A university student majoring in public health who studied vaccinations in children for a research paper will be better informed (and possibly more likely to respond) than someone who is not familiar with the debate on vaccinating children.