There traditionally aren't a lot of happy endings to shows, which I'm sure is up for debate, so it's refreshing for a lot of people I'd imagine. Married With Children & Roseanne had continued suffering, MASH & Cheers were bittersweet, That 70's Show & 2 1/2 Men suffered from actors fucking up the storylines by ducking out. If you wanted to praise something about Big Bang Theory, it's that 4 unlikeable nerds start out by taking their beatings, and life eventually gets better for them as time goes on, and that's a vibe people can really relate to. The marriage arcs lead to geeky guys getting women way out of their leagues, and that hits more people in the feels than will publicly admit.