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John SMy office mate, who has no interest in collecting anything, asked me this question yesterday.  I know this topic has been discussed a number of times in a multiple threads.  Beyond the superficial answers (love of baseball/history etc., money for some) are deeper explanations.  Throughout my life I have found myself collecting just about anything.  Feathers when I was very young, rocks, coins, stamps, biological specimens (I have a permit!), and of course cards. The list goes on and on.
I have actually done reading in some psychology and animal behavior texts regarding collecting.  Some neurotic individuals with collecting compulsions (my wife sometimes questions if I should be placed in this category) have been found with dust-bunny, fingernail, tacks and other items that most sane individuals would not dream of collecting.
Primal explanations for collecting include success as hunter/gatherer and the need to categorize and organize as a way of making sense of one's environment.  
My office mate presents another question.  Why do some individuals have no interest in collecting at all?  Maybe I am thinking a bit to deeply this morning, but I am interested in responses from fellow collectors.