Obviously a personal anecdote and YMMV, but I have several nieces and nephews, and I've gotten to watch them grow over the last ~15 years. During this time, I've tried to spend as much time with them as I can.

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.