1. An interactive program search apparatus which searches out a program using a search word, said apparatus comprising: a program information storage unit configured to store program information indicating contents of programs on a per program basis; an association dictionary storage unit configured to store an association dictionary which indicates an association and a degree of relevance between words included in the program information; an obtaining unit configured to obtain the search word; an obtainment history storage unit configured to store obtainment history information which includes the search word obtained by said obtaining unit and an obtainment frequency of the search word in association with each other; a program search unit configured to search out programs from the program information using, as a search condition, the search word included in the obtainment history information; an association-source word extracting unit configured to extract an association-source word which is associated with an associated word, based on the obtainment frequency included in the obtainment history information stored in said obtainment history storage unit, the association-source word being extracted from among the words included in the obtainment history information; an associated word extracting unit configured to extract the associated word associated with the association-source word using the association dictionary; a reason word extracting unit configured to extract, as a reason word indicating a relationship between the association-source word and the associated word, a word included in the obtainment history information from among words each having (i) a degree of relevance equal to or greater than a predetermined degree of relevance with the association-source word and (ii) a degree of relevance equal to or greater than a predetermined degree of relevance with the associated word; and a display unit configured to display program search results which are the programs searched out by said program search unit, the associated word, and the reason word.
5. The interactive program search apparatus according to claim 1 , wherein said reason word extracting unit is configured to extract words as reason word candidates from the association dictionary using the association-source word, the associated word, and the obtainment history information, and determine the reason word from among the extracted reason word candidates, based on the number of programs including the association-source word, the number of programs including the associated word, and the numbers of programs including the respective reason word candidates, the numbers of the programs being determined with reference to the program information stored in said program information storage unit.