Jamieson, Fausset and Brown on Genesis 24:3

3. thou shalt not take a wife, &c.--Among pastoral tribes the matrimonial arrangements are made by the parents, and a youth must marry, not among strangers, but in his own tribe--custom giving him a claim, which is seldom or never resisted, to the hand of his first cousin. But Abraham had a far higher motive--a fear lest, if his son married into a Canaanitish family, he might be gradually led away from the true God.