Since comic books tend to be like tv dramas; having a flowing and continual plotline with dynamic characters, many people, mostly a younger audience; from grade school students through college kids, follow their periodic- usually monthly (but weekly for big-name characters)- exploits. This leads to the keeping of these books, and after so long those who did not keep these books from their childhood want them back, why that is no psychologist can wholly agree upon, but nonetheless they want the things of their childhood back and are willing to pay.
That is how it started, but now it has boomed into the companies themselves releasing alternative versions of popular books, limited-editions, and other gimmicks to entice more buyers. Some collectors may buy five copies of the same story content, just because the book has a different cover. They do this in the belief that the product will be in demand, and they can resell for a profit. With the coming of the 'internet-age' there has been a big interest in comic books,or at least an increased level of contact between collectors; sellers and buyers. Online auction sites, like eBay and Amazon.com are benefiting from this increased interest.
Comic book collecting is like all other collecting; some collect for-profit, others for memories, but all are drawn to the vivid colors, bright illistrations, and colorful characters of the comic book universe.
The period of time from roughly 1985 through 1993 (? - please add correct date) is seen as the point where comic book speculation reached its peak. Beginning with the publication of The Dark Knight Returns and the beginning of the "summer crossover epics" with Crisis on Infinite Earths and Secret Wars, comic book publishers began producing a high number of comic books designed especially for the collectors' market. [Mention of marketing gimmicks, such as multi-cover issues]] The market reached a saturation point and finally collapsed during the middle of the 1990s. (Was there a certain event seen as the trigger for the collapse?)