First of all, you simply won't get it if you only have a firstborn
Second, it's not about the child actually being more precious than any subsequent children. I love all 3 of mine equally but I can not deny that DS1 was treated far more preciously than DS2 or DD.
Some examples:
DS1: I would have passionately fought off any child or whatever age who came anywhere near him as a baby.
DS2: I smiled indulgently as DS1 and friends investigated/played with him
DD: I let her get poked and prodded by an entire class of 30 reception age children without batting an eyelid.
DS1: Always clean, nappy-changed and properly dressed and fed before going anywhere
DS2: Sometimes changed/fed/dressed when we arrived somewhere rather than before we left to go there
DD: Don't always rememeber to check what state she's in before going awywhere.
It's about little things that you look back on and laugh at yourself when/if you have a subsequent child. When I only had DS1 I would have sworn I was laid back and calm about stuff (and I was compared to friends etc). Then I had DS2. Then I had DD and realised just how neglectful you can be and they'll still survive
It's also something you must never, ever point out to a parent of a PFB if you spot typical behaviour. You just have to smile to yourself and remember how it used to be... Sometimes I hanker after the simplicity of my PFB days. Sigh.