Why is the blessing hatov v'hametiv recited only on wine?
and why is it said only in a case where a second type of wine is brought.
looking for the message behind the halacha
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This yeshiva.org.il article suggests an answer to your question:
This is because wine is unique in that not only does it satiate, it also gladdens the heart. In addition, each type of wine has its own unique character, and when additional types of wine are consumed in company there is greater joy. This is why the sages instituted a special blessing over the consumption of additional types of wine (Berachot 59b; Tosefot and Rosh ad loc.).
While the rabbinic understanding is sourced in Talmud Bavli Berachot 35b, it is based on the specific biblical (albeit, Ketuvim) application of 'to gladden/be happy' found in Psalms 104:15.