In feudal societies like Westeros, why is romantic love, well, romanticized to the extent that it is when the system is built on political marriage? Wouldn’t the idea of marrying for love be stigmatized to a greater extent?

Well, in feudal societies romance isn’t linked to marriage. In Lancelot and Guinevere, Tristan and Isolde, and all of the “courtly love” formulas that flowed out from them, the central figures are a (usually unmarried) knight and a married or otherwise unavailable lady, love from afar is valorized as pure and noble whereas consummation leads to death and dishonor. In this way, the potential threat that romantic desire poses to arranged marriages is contained.

In this context, marriage for love is also a threat to the social order, because it tends to involve broken betrothals, inappropriate matches, and subsequent poverty and loss of estate. Hence why there was an emphasis on intense passion as dangerous – thus why so many famous love stories involve the lovers dying rather than living happily ever after – whereas contentment and stability are praised as harmonious.