This is part of a series of questions on the Gemara in Bechoros 8b-9a. Below is a summary of the relevant parts of the Gemara. After being challenged by Caesar, R' Yehoshua went to Athens to defeat the Athenian Elders in a battle of wits.
In the process of their debate, the following exchange occurred.
"Say that a man wants to marry a woman," the Elders asked him, "and her family refused to give her to him. Why should he then go ahead and attempt to marry a woman of a better lineage?"
R' Yehoshua picked up a peg and stuck it against the lower part of the wall, and it wouldn't stick. He stuck it against a higher part of the wall, and it stuck. He explained, "Maybe he wasn't meant to marry the woman of the lesser lineage; certainly he should attempt to marry the woman of the better lineage, for perhaps he is destined to marry her."
Now, the Elders' question is on the presumption that a family with better yichus would surely prevent their daughter's marriage if the family with worse yichus refused him. Why? The family in their case never specified that they aren't giving her to him because he's not good enough for them.