His father Abraham leaves him a huge pile of money and makes his trusted servent Eliezer, swear on his (Abraham's) old, wrinkled testicles, to find the perfect girl for Isaac, who is by now nearly forty. Eliezer find this beautiful, kind and hard working three year old, Rebecca. After some negotiating Eliezer gets Rebecca away from her no account family and brings her back to Isaac who is out in field doing... G-d knows what.
Rebecca is so shocked when she sees her goofball betrothed she nearly falls off her camel. But Isaac loves her. He moves Rebecca into his mommy's tent and they become husband and wife.We know that later Rebecca is clearly the brains of the family and ends up manipulating Isaac in a most crude way. Nonetheless as the Torah's most precocious little girl she is the ideal match for a mentally challenged middle aged man.
At least that's the way it is when WE tell the story. - SHABBAT SHALOM! - a & s