Your enthusiasm becomes their enthusiasm; your lukewarm presentation becomes their lukewarm interest in what you're offering.

My theory is that, just like with omitting a final comma in a list when not essential for meaning, publishers are trying to save paper and ink or pixels on-screen.

The dash helps to indicate that the two thoughts are intimately related, and it's less stodgy than a semicolon, which would have performed the same function (and who talks in semicolons?).

I have a sense of humor. I usually come off as very serious, but I definitely have a dry sense of humor.

We all know gifs are pronounced "jifs," right? Their creator says so, damn it!

Your enthusiasm becomes their enthusiasm; your lukewarm presentation becomes their lukewarm interest in what you're offering.

I caution against beginning or ending a quotation with ellipses