Shortly before his marriage, Yitzchak had reached a remarkable degree of spiritual perfection. Right at the beginning of his life he was the first Jew to be circumcised at eight days. He was then educated by Avraham our father, and later showed an eagerness to sacrifice his life to G-d, at the Akeida, from which point on he attained the sanctity of a burnt offering [an olah temimah].
Rivkah, on the other hand, was ''a rose amongst the thorns'', born into a wicked, idol-worshipping family.
The union of Yitzchak and Rivkah was thus a meeting of extremes, and for this very reason it is recorded in the Torah, since Torah itself is a guide to uniting extremes. For when any mitzvah is observed, a mundane physical object becomes infused with Godliness and holiness.
Thus, the marriage of Yitzchak and Rivkah represents the marriage of the spiritual and the physical. And this explains why the Parsha spends so much time discussing their story, since it was the basis of everything that was to follow.
Source: Lubavitcher Rebbe: Based on Likutei Sochos vol 20 p 95-96