Associative nature of thought
Thought about a certain concept is influenced strongly by (possibly weak) associations with other concepts.
Physiological reason
Neurons which fire together wire together - they are highly social. Nature of human thought is associative.
Examples
People, when asked to think about the past or future, tend to lean slightly back or forward. People when exposed to a crossword with polite words tend to exhibit more polite behavior. People asked: ‘Which continent is Kenya in?’ and ‘What are the opposing colors in chess?’, reply ‘Zebra’ when asked to name the first animal which comes to mind. When asked ‘What does a cow drink?’, the first word which comes to mind is milk. Atheists exposed to the 10 commandments are more likely to exhibit moral behavior. People exhibit more moral behavior when in the presence of pictures of eyes/ faces.