Where did the word "rub" originate from?

Isn't "the" necessary in front of "most" here?

Great question. "Most" can be used as both a superlative and an intensifier. In this case it is being used as a superlative, meaning of all the cities under consideration, this one has the greatest number. In formal usage, you would almost always want to use the definite article the with a superlative, the exception bring for possessive constructions, which is the case here. Because we are using "Canada's", a possessive form, it would be grammatically incorrect to put "the" before "most". Ex: John's most important job is filing. Ex: The most important job is filing. Note that in conversation, you may often notice that native speakers leave out "the" but remember that it is implied Ex: I like this one [the] most.

Where did the word "rub" originate from?

Firstly "here's the rub" is an idiom meaning here is the biggest problem or difficulty which will be difficult or impossible to deal with. "The rub" is a reference to the English game of bowls, where a rub is a flaw in the playing surface that interferes with the ball’s route. So in this case, "rub" is a difficulty or problem. The phrase is now a common British English idiom after it was famously used in Shakespeare's Hamlet (as "there's the rub") and is now seen in many different variations, including "therein lies the rub" and "that's the rub" or as we see here "here's the rub". Ex: I just have to pay the fine to get my car back, but here's the rub, my wallet is in the car. Ex: You can't get a job unless you have experience. And here's the rub – how do you get experience if you can't get a job?

Is it common in western culture to view a building as something that wouldn't last forever?

No, it's not common to view a building as something that doesn't last forever. So here, he's actually trying to be funny by reversing the two things! A greeting card is definitely something that does not last forever, and will disintegrate much faster than a building would. Ex: I don't like getting cards from people because I always lose them or throw them away. Ex: This building has been here for over 100 years.

What does "first glance" mean? Is there a expression of "second glance"?

"First glance" means to look at something for the first time without noticing any details. To answer your second question no, there is not a "second glance" expression. Ex: At first glance, the apartment looked perfect, but when we looked closer, we noticed so many problems. Ex: At first glance everything seemed fine. Ex: The job seems wonderful at first glance until you speak to the employees.

What is "buffalo wings"?

Buffalo wings are a type of food commonly found in American restaurants and bars. It consists of deep-fried chicken wings coated with a type of sweet, spicy sauce. Ex. Every time I go to the pub, I order buffalo wings and a pitcher of beer.