This may be just me, but when I ask this question of my boyfriend, I actually want him to tell me whether the dress makes me look fat!
I have a certain body image in my mind, that unfortunately got stuck when I was around sixteen years old and several kilos lighter. Whenever I look into a fullsize mirror I am struck by how much I resemble my mother nowadays (this is not a good thing). Automatically any item of clothing that seemed fabulous on me in my mind's eye loses much of it's appeal in the harsh light of the store.
My mental body image also greatly varies with my mood. When I'd just found out my leather trousers didn't fit me anymore because I'd gained weight, I couldn't shop for at least a week because everything looked horrible on that horrendously fat body with the stupid way-too-short legs. When in love my body image is a lot better. Then I tend to buy clothes that are not really 'my thing', because when I feel great I think I look great in anything. (I don't. Trust me.)
Because of all this, when I try on something new, I need someone with me who has a more stable view of me and what suits me. Here's where the boyfriend comes in, and gets asked the horrid question: Does this make me look fat? If so, the dress goes. If not, the dress gets to be bought. There is no right answer, just a truthful one.
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If however someone gave me the answer kslawson provides above, I would be sure the person in question found me VERY fat but just didn't want to admit it.