The Netziv writes that the area of zivugim is “kavshei Rachamana,” among G-d’s secrets. We might look at Ploni and Plonis and wonder what such different people ever saw in each other, yet despite our preconceptions and misconceptions they remain happily married for years. We might look at another Ploni and Plonis and think they would make the perfect couple, yet after one date they don’t even want to hear each other’s voice. In short, G-d in his infinite wisdom works things out in ways that we sometimes cannot anticipate. Where Ploni and Plonis come from different families and live in different places, the hand of hashgacha is more obvious. A guy from Australia is assigned a roommate in yeshiva who happens to have a sister in Chicago whose friend from Detrot ends up being his bashert – only hashgacha pratis could pull strands from all over the globe together to make such a match. But when Ploni and Plonis come from the same family, like Rivka and Yitzchak, the hand of hashgacha is not obvious. The family already has a connection, the match already looks like one that is appropriate, it’s no surprise for the two to come together. Yet, even Lavan and family knew that what looks like the perfect match is not always the right thing. Even they understood that it’s the yad Hashem that is the true confirmation that the match will work.
2)The Derashos haRan famously explains that the reason Avraham look for a bride for Yitzchak from his family instead of from Canaan, even though both were idolators, is because the Cannanites had corrupt midos while his family just were misguided in their deyos, their beliefs. A philosophy or belief system can be changed; midos, however, are genetic, and the corruption would inevitably pass to the next generation. Taken at face value it’s a hard sevara to understand. Midos, like beliefs, are not inherited characteristics. That being said, I think most people would agree that there is a distinction between the two. R' Yisrael Salanter's remark about it being easier to learn shas than to fix a midah points to the truth that ideas are far less fixed than behaviors are.
The Ksav Sofer offers a different reason based on a diyuk in Avraham's command to Eliezer not to take “m’bnos Canaan asher anochi yosheiv b’kirbo,” “a girl from the Canaanites among whom I am living.” Why did Avraham need to mention that he lived among the Canaanites -- we know this is true? Ksav Sofer writes that Avraham was justifying his rejection of a Canaanite girl. For decades Avraham had lived among the people of Canaan and tried to teach them Hashem echad, not to worship idols, etc., yet, despite all his efforts, they remained who they were – idol worshippers. If after all those years lving among them his teaching and his example had no effect, there was no reason to think a girl from a Canaanite background would make a good shidduch. Avraham therefore had to look elsewhere.