This may sound somewhat reductive, and please correct me if that is the case, but I find that I can be around and play with my nephew and experience all of this wonder and how he progresses through life, while not being around for the sleepless nights and tantrums that I feel are the "work" part of being a parent.
But I'm a new dad as of this year, and it's totally different. It really doesn't compare.
There's only so much time you can spend with someone else's kid, even if they're family. Even seeing a niece or nephew every week doesn't compare with waking up with a newborn at 3 AM and seeing them stick their tongue out for the first time, or seeing the squinty face they make when they try a strawberry for the first time at 5 PM on a random Tuesday.
In a sense, one could say there's no greater joy.
But there's also perhaps no greater pain when parenting gets really complex and hard. One of my kids is autistic and raising him has been incredibly difficult and painful (he's a teen now).
Edit: I think another poster put it well that “it wasn’t about ‘me’ anymore”. Depending on your disposition, this could be great or it could be really bad. I’ve definitely changed for the better over the course of being a parent.
No, not really.
Also not the same as being the cool uncle/aunt, though that can give you a lightweight preview. I know because I went through this myself, I was so involved with my nephews I became their favorite uncle by far, and to this day they still remind me of this.
In reality, only being an actual parent can give you the true experience. Nothing else comes close.