Dean has travelled the world many times; he has seen many women and known many women, in the Biblical sense, and here is something that they all had in common: they were not Emily.
Dean has travelled the world a great deal, and he has seen a great deal that Emily is not.
For example, he has seen a painting of a woman with a smile so maddening that learned men have written reams and reams about how it was done and what she was thinking when it was done and never come up with a satisfactory answer. Her smile is not like Emily's. If Dean wants to see a smile like Emily's, all he has to do is discover a shop in Paris which imports Canadian magazines and find out which of them the great Theodore Kent has deigned to be published by this week. If Dean ever meets the great Theodore Kent (which he can't ever remember doing, but he must have done at some point. Impossible, in that godforsaken hole Blair Water, not to have met someone) he's going to pop him one right in the eye for using Emily's smile without his permission. As if he could. Theodore Kent is probably a lithe, athletic young man, or Emily wouldn't be so fascinated by him and not Jarback Priest.
When Dean's in Paris, all he has to is walk to a park and suddenly he is inundated with memories of Emily, her face shaded by ancient cedars. In point of fact, when Dean is anywhere, all he needs to do is close his eyes and he can see her, perhaps laughing at the foibles of a neighbour (because while Dean knows to his cost that Emily is a good girl, she is not a nice one, not always). Wherever he is. It's only that it's worse when he's in a place that reminds him of the garden at New Moon, peaceful and green with summer.
This is the real reason why Dean will not return to Blair Water. In Paris, he is only homesick; Canada would make him heartsick.