I recently came across this chart from bab.La and immediately noticed a very telling theme. I am not surprised by the theme, just fascinated to see it illustrated. (click on image to enlarge)
By my count, 19 of the 100 most listened to English words and expressions contain a th-sound, either word-initially as with "thanks", word-internally as with "mother" or word-finally as with "earth".
The reason I am not surprised by the number of words and expressions that contain the th-sound is that th-sound is notoriously challenging for ESL students to learn. Indeed, even native English speakers struggle to acquire the sound as children.
Phonetically, the th-sound is articulated either as a voiced dental fricative called "eth" and transcribed as or a voiceless dental fricative called "theta" and transcribed as The eth is heard in the word "though" and the theta is heard in the word "through".
Not only are both phonemes hard to learn, they are also hard to differentiate. Check out this previous post for more about theta and eth.
The bab.La site also has an R-rated version of the the 100 most listened to English words and expressions that includes many thetas and eths.