I don't know what's wrong with the common sense of the people of these days but in the basic narrative two best friends of the opposite genders are automatically endgame, they are designed with that idea as a method to give deep to the relationship, Beatrice and Wirt has many scenes that are very common to create a romantic relationship between two characters, for example being locked in a closet forcing both of them to get closer.
Even when both characters are really worked they follow a typical development about two people of the opposite gender whom don't like each other but slowly they learn to respect each other, got closer and eventually develop feelings for each other. There is a romanticism around some Wirt and Beatrice's interactions and scenes such as during the episode 6 where we can see them together while the many pairs share a romantic moment.
Wirt considers to Beatrice something unique as in the same episode "Adelaide's Parade" song takes another connotation "I don't know who she is or how she is or how and why but if she is there we will go there, to Adelaide", in this episode Wirt's doing an allusion to Beatrice and McHale explained that this episode was supposed to be two episodes being the second one about the woodsman trying to prevent to Wirt of go to Adelaide's house but Wirt deciding to go anyway because he thinks that Beatrice is in danger and he wants to rescue. We can describe Wirt's opinion of Beatrice with a reference to The Divine Comedy "She is a force that motive him to be a better person".
At the same time Beatrice considers to Wirt unique as she considers that his good qualities are better than his flaws "Wonderful mistake of nature".
Many people don't understand For Sara because it's not easy as the author keep using metaphor and references all the time, but yeah, Wirt talks about Beatrice in them a lot.