i get that a lot when i am cooking, i suspect that often you are just fed up of looking at it and the appetite dwindles. as said above, when it is a surprise delievered on a plate the senses are overloaded, but after staring at the gloop in a pan for half an hour its only natural to feel a bit deflated at first.
say you bought yourself a donut and then sat looking at it for 30 minutes. you will still eat it of course - as you do with the food you cook - but during that 30 minutes your mind would have been putting you off the idea.