I think the sheer escapist potential of spirit/matter-dualism contributes greatly to its appeal, but I don't think that's how people first get to believe in it.
If you ask me, the whole notion is based on our ability to create, think and communicate in terms of symbols. Once we're out of our earliest infancy, our whole conception of reality is processed and contained within the symbolical sphere of language and related sign systems. (Which is also why we have a hard time accessing our earliest memories: they're basically stored in another "format".)
Now, the history of dualist thought probably started with the ancient Greeks, and their adaptation of phoenician letters. Up until that point, letters had never been mere phonetic units, but usually referred to a HUGE semantic field of symbolical meanings. (Just look at the way each Hebrew letter is actually a word in itself, or the way Hieroglyphs work, or even Chinese pictograms and their way of creating semantic fields around the term they describe.)
Greek script was divorced from that: it consisted of "pure" symbols that were not directly connected to physical objects or the like. So Plato and people who thought like him figured that abstractions must be MORE real than the physical reality that surrounded them. (He actually hinted at the notion that our perceptions shape our realities, and that languages do much more than just describing what is "out there".)
Of course, Plato's concept of the "ideal" needn't be dualistic, unlike René Descartes nonsensical remonstrations.
Likewise, there are materialists out there who are basically dualists in disguise - except that they basically amputate the "spiritual" altogether, and insist that the "material" half of the dualist equation is all that's there.
Both the Cartesian and the materialist miss out on the Big Picture, if you ask me: in Buddhist terms, I'd accuse them of seeking permanence in impermanence (and missing the TRUE, atemporal sense of permanence contained within each moment). They spend so much time worrying about the fact that flowers are bound to wilt that they can't appreciate the blooming that's happening right in front of their eyes.