Why did Yaakov prefer Rachel?

The answer, I think, is that the heart wants what the heart wants. And perhaps that it was love at first sight (at the well, but that is admittedly debatable). And that she was the prettier sister:

טז וּלְלָבָן, שְׁתֵּי בָנוֹת: שֵׁם הַגְּדֹלָה לֵאָה, וְשֵׁם הַקְּטַנָּה רָחֵל.	16 Now Laban had two daughters: the name of the elder was Leah, and the name of the younger was Rachel.
יז וְעֵינֵי לֵאָה, רַכּוֹת; וְרָחֵל, הָיְתָה, יְפַת-תֹּאַר, וִיפַת מַרְאֶה.	17 And Leah's eyes were weak; but Rachel was of beautiful form and fair to look upon.
יח וַיֶּאֱהַב יַעֲקֹב, אֶת-רָחֵל; וַיֹּאמֶר, אֶעֱבָדְךָ שֶׁבַע שָׁנִים, בְּרָחֵל בִּתְּךָ, הַקְּטַנָּה.	18 And Jacob loved Rachel; and he said: 'I will serve thee seven years for Rachel thy younger daughter.'

unless rakkot means soft, as in beautiful. Perhaps the juxtaposition means nothing, but it seems like a deliberate juxtaposition. She was the prettier sister, and Yaakov fell in love with her. I can understand, though, how this idea might make more conservative people squirm. Is Yaakov straying after his eyes? Shouldn't he look to her middot tovot? What is a tzaddik and a prophet focusing on such matters? Sheker hachain vehevel hayofi!

Ralbag admits at the end that this was a partial consideration, but he also first gives a few other reasons for Yaakov's preference.

It is not that Leah is chas veshalom not pretty, because of her tearing eyes. Rather, since they were weak and teary, which reflects a certain sickness, he did not choose the older one, Leah. And he chose the younger one, Rachel, so that the children he would have from her would be stronger and more complete. And combine to this, of course, that she was very pretty. And so Yaakov said to Lavan that he would work for seven years if he gave him Rachel his daughter, and Lavan desired this arrangement.

While Ralbag reads these ideas into the words of pesukim, I think he is reading in meaning rather than discovering inherent meaning. And the calculations about what sort of children he would get from Leah vs. Rachel seems somewhat crude, or rather unromantic.

(Related to this, the question from two weeks ago why the Torah would bother saying something so trivial about Rivkah that she was very pretty. Why should this matter? And various meforshim grapple with this, though Ibn Caspi asserts that this was why Eliezer ran to her.)

That the avos were (also) human beings, and weren't always motivated by halachic concerns or deep spiritual reasons seems like a rather strong possibility. And it is not a bad thing for one to be attracted to one's spouse, and to make this into one consideration among others.

Related: See my post about Rabbenu Bachya, and whether Yaakov strayed after his eyes in preferring Rachel.