That's the simple truth. I want THAT particular item. Of course, this desire is not completely irrespective of the price asked, but Steve has reminded us that while one person may throw it out, another may assign it value.
I purchase two principal categories of items: books and memorabilia. The former I see as mind-expanding and the latter nourishes my sentimentality. Both of these categories often see me bidding in excess of "market" value (whatever that means), because the items in question occupy a strong place in my heart and mind. As such, and combined with my sniping stragtegy, I tend to win most of my auctions.
As I have posted here before, I am of the "set your snipe amount and walk away" school of thought. This is an integral part of my sniping strategy. In so doing, I set a high enough amount that allows me to overcome most nibblers. Occasionally I do not, and I accept this as proof that someone was willing to pay more--and probably had the same desire to nourish his mind or heart, for which I cannot blame him.
Occasionally I do chuckle when I see a bidding war in which I do not participate, or moronic nibblers who nickel-and-dime the price up, even while they continue to bid pittances. I realize that in some auctions the seller ends up getting a lower final price because of my snipe (in that the nibblers may have nibbled past my max, had I bid earlier), while other times I realize that the seller gets more (usually when there was no bidding war going on and my snipe simply raised the final price higher, or where there were no prior bids at all).
But the REAL reason I snipe is because because I like to sign on to eBay and see that I have won my precious widget, period!