Both the German and French languages, along with English, evolved from the same roots. This is reflected in some of their words and grammatical structures. So then why are the pronunciations of both languages so different? Why is German so harsh with more emphasis on rough sounding letter combinations (st, sh, tz, ch) as compared to French (which I know is a Romance Language but have no clue as to what it means)?
The short answer is, sounds change really fast. Compare different regional accents in English (Southern, East Coast, Australian, Indian)—outside of Britain, all of these differences have arisen in the past two hundred years or so. Or look at some of Shakespeare's rhymes, like "love" and "prove", or "wind" and "kind", which rhymed perfectly in his day.
The long answer is, it's been a long time since the family tree of French and German split apart. By most estimates, their last common ancestor (Proto-Indo-European) broke apart over five thousand years ago. One branch became Germanic which eventually evolved into German (and Dutch and Icelandic and English and…), while another became Italic, evolving into Latin, then French (and Spanish and Romanian and Italian and…).
Now, French and German are spoken in close proximity, and so they're having an effect on each other—but they've been close for a relatively small amount of time. It's entirely plausible that in time Standard German will gain nasal vowels and Standard French affricates, but at present that hasn't happened.
As a side note, the "harshness" or "roughness" of a language is very difficult to measure. You say the "sh" sound is harsh, but what makes German Schiff "ship" any harsher than French chat "cat"? They start with the same sound. A lot of it comes down to individual perceptions rather than anything quantifiable.