Which is an awful lot of pressure. Not for me - I eschewed the whole process (compelled to by my bodily bulk - bridal shops stock samples in sizes 10-14 only), but for more pragmatic brides. Anyway, I thought I'd share my thoughts on the matter:
[The One is] a concept that works because of the way we buy dresses.
You try on a selection of dresses - it's obvious that certain styles will suit your body or your taste more than others. It may be that the one that suits and flatters you best is the first one, or the eighth.
You may then visit another shop - if you haven't found a dress that truly appeals, you'll go looking for something which does. if you have found a dress you love, you'll be less likely to give the others a chance.
Often people talk themselves out of the one they liked best because of price: but it stands to reason that pricier dresses will be made in lovelier fabrics and with more attention to design. Therefore they prey on our minds as we try on other dresses. Or it may be that our emotional state when we try on certain dresses is more conducive to our liking ourself in the frock. Or it may just be that any given sample fit better or was more to our taste.
The point i'm making, not terribly well, is that if we could try on all the dresses in the market on the same day and with the same mindset, we may well not end up with the dress we perceive to be 'the one'. But wedding dress shopping is a limited exercise - rather like choosing a life mate, no?
Ultimately, then, one's chance of finding a dress we like more than all the others we have tried on is absolute (assuming we try on more than one). The chances of us feeling strongly positive toward any one dress depends on the section of the market we experience happening to encompass a dress which actively appeals to our personal ideas of taste and suitability. This in turn depends on a person's preconceptions going in to the process (i will look good/stupid in a dress), and how well the market (in section or whole) caters to that individual's body shape, taste and budget.
It's just like finding a life partner!