Well...every mouthpiece has a specifically shaped curve where the reed is placed (table, then the rails to the left and the right of the window, then the tip).
Now, some mouthpieces are curvier than others, and some reeds are stiffer than others. Now you'll understand that it takes more force to "bend" a stiff reed around a narrow curve than around a flat one. In the same light, a flexible reed might close up too easily around a flat, shallow curve.
So, there is only a certain bandwidth of reed stiffness that is well suited for a certain mouthpiece.
There is nothing inherently "good" or "bad" about a soft or a stiff reed. First it must be a good combination with the mouthpiece, but also the player's embouchure (how firmly the player closes his lips around the mouthpiece) and the style of music.
You should "work up" to a reed strength you're comfortable with, you can play for extended periods of time without getting a tired mouth, and last but not least produce a satisfying tone and are in tune with others. That should be your goal, not some meaningless competition about who's playing the hardest reed.
Over time you'll find out that you have changed, and so also your requirements re the reeds will change. That is normal, and in the end everyone will gravitate around a certain reed strength but will eventually go a notch softer or stiffer.
(This all in layperson's terms, I don't mean it to be scientific in a way)
Oh, and one factor is very, very important: Playing is supposed to be fun.