This phenomenon is called the endowment effect. Once something is given to you, it's yours. Once it becomes part of your endowment, even after a very few minutes, giving it up will entail a loss. And, as prospect theory tells us, because losses are more bad than gains are good, the mug or pen with which you have been "endowed" is worth more to you than it is to a potential trading partner. And "losing" {giving up} the pen will hurt worse than "gaining" {trading for} the mug will give pleasure. Thus, you won't make the trade.
( Barry Schwartz )
[ The Paradox of Choice: Why ]
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