Why does the digestive enzyme pepsin need to “enjoy” acidity

Essentially, the optimal pH for pepsin enzyme activity is an ACIDIC pH, because this acidic environment is what activates pepsin. So the question now is, how does an acidic pH activate pepsin? Well...

If you didn't already know, pepsin is activated from pepsinogen (inactive form of pepsin) AND HCl (hydrochloric acid): Pepsinogen + HCl = Pepsin (enzyme)

So when you have new food in your stomach, it's time for digestion, so parietal cells secrete HCl, and this HCl provides an acidic environment that tells the stomach "it's time to release the enzyme pepsin for digestion" , but in order to release pepsin the stomach has to first convert pepsinogen (inactive precursor) to pepsin. So therefore, the two ingredients needed for pepsin activation, release, however you want to call it, is PEPSINOGEN, and especially, HCl.

The chief cells in your stomach secrete the peptide PEPSINOGEN, which is inactive. It needs hydrochloric acid, secreted by your parietal cells, to be converted into its active form, PEPSIN, inside the stomach.
Hydrochloric acid creates an acidic environment, which allows pepsinogen to unfold and cleave itself in an autocatalytic fashion, thereby generating pepsin (the active form). Pepsin cleaves the 44 amino acids from pepsinogen to create more pepsin and so on.

So basically it is bound to enjoy acid as programmed genetically.