Or HaChaim on Genesis 24:1/אור החיים על בראשית כ״ד:א׳

1 ואברהם זקן, And Abraham was old, etc. Inasmuch as Abraham felt that he had aged and also in view of the fact that G'd had greatly blessed him, he worried that the servant who was in charge of all this wealth while his master was visibly aging would develop notions of marrying his daughter to Isaac. Eliezer might want to secure Abraham's wealth for himself through the marriage of his daughter to Abraham's son. This is the reason Abraham made Eliezer swear an oath concerning the fact that his son was not to marry someone of Canaanite descent. Eliezer would thus get the message that his daughter was not suitable marriage material for a son of Abraham who was blessed whereas the Canaanites were cursed (compare Bereshit Rabbah 59,9).