Pavlov discovered that he could teach dogs to salivate at the sound of a tone if he repeatedly paired the tone with the presentation of food. An obvious example is the emotion of fear.
People often learn to fear things because of previous unpleasant associations.
A woman who has been attacked may develop a fear of strangers or a fear of men. Or maybe it's the garage or neighborhood in which she was attacked that come to elicit feelings of fear and anxiety. On the positive side, think about an old girlfriend of yours that you adored. You were crazy about this girl and would have done anything for her. As a result you may have done a few things you didn't really care too much for.