In auction theory, why is my own valuation a random variable? Auction theory typically (always?) begins by assuming that each bidder's valuation is a random variable. Now, it might seem reasonable (at least from a Bayesian perspective) for you to treat other people's valuations as random variables. After all, you don't know their valuations! However, what justification can there be for treating your own valuation as a random variable when deciding on how to bid? And are there any approaches which do not begin with this assumption?

Edit: Here's another way of posing the question. In the context of auction theory, people normally seem to define a strategy as a function mapping from my (random) valuation to my bid. But assuming that I know my valuation, why not simply define my strategy as my bid?
In most of the literature I have read on private value auctions you do actually know your own valuation in the auction, it is your private information. It has a distribution from the point of view of others, who can only guess at your valuation.

Another interpretation would be that your valuation is actually random before getting detailed information on the object you are bidding on. How much would you pay for this bottle of wine that I have here on my shelf? Oh, it depends on the wine? Then without knowing the exact type of wine and assuming it has some random distribution your valuation is also a random variable. Once you learn the exact type of the wine you learn your own 'type', which is modeled as getting a private signal about your valuation. 

In case of noisy observation your valuation can actually be random. E.g. I tell you the type of wine but you have never had it before. Or in case of gas companies, they have an inexact estimate about the yield of a gas field. Their 'real' valuation depends on the 'real' yield, without knowing that they can only bid based on some expected values.