One of the reasons we buy new things is that we become bored with what we have. The object that at one time was novel transitions to mundane. On successive occasions of having a particular experience, Gilbert writes, "we quickly begin to adapt to it, and the experience yields less pleasure each time. Psychologists call this habituation, economists call it declining marginal utility." He suggests there are two mechanisms to counteract habituation - variety and time, although variety isn't necessary if there is sufficient time separating experiences.
Although Gilbert doesn't make the connection explicitly, presentism, or the tendency for current experience to influence one's views of the past and the future, also seems to play a role in stimulating purchases. Upon an initial novel experience with an object, we assume we will continue to have the same positive feelings for long after we purchase it - forgetting to take habituation into account. Gilbert does propose that comparisons have a similar effect. When considering the purchase of a new item, we compare it to our current, habituated, mundane possession - rendering the new, novel and/or the current, outdated. However, we fail to consider that after purchase and the discarding of the old, the delight produced by that initial contrast is gone. Styling, and perceived obsolescence, takes advantage of this phenomenon to persuade essentially unnecessary upgrade purchases.
Gilbert's theories don't sufficiently explain the existence of favorite, long-standing, frequently used possessions, as he doesn't account for contained memory, attachment, and meaning in objects. However, Michael Pollan, in Omnivore's Dilemma, proposes a framework for eating that transfers well to objects. (One of many concepts in this book that translates smoothly into material culture.) He suggests humans' "innate neophilia - the pleasure of variety, and neophobia - the comfort of the familiar," motivate the contradictory inclinations for the new and the known. We sometimes seek the excitement of novel experiences and stimulations and other times prefer the safe reassurance of the familiar.