I had the complete opposite feeling when my brother became a father and thus I an uncle.
I was selfishly sad at the fact that I'll barely be able to hang out with my brother anymore and that turned out to be true. Parenting is exhausting and eats up all of your free time. About twice a month he could get out of the house to have around 90 minutes to hang out. This was nice and all but it's just not the same.
In addition to the realities of being a parent I hate kids for the lack of a proper term. I get anxious, stressed and uncomfortable around them even when they aren't interacting with me. Just being at their house with kids mostly in the next room will typically lead to headaches within an hour and I just feel worse the longer I'm there. Natrually it feels even worse when they want to interact with me.
My family understands how I feel about it and they begrudgingly accept this for the most part. They still try to push child interaction upon me but they know better than to ever ask me to babysit or even watch the kids for a few minutes which is a good balance I suppose.
For me, life was WAY better before becoming an uncle. But again, that's just my selfish take on the situation and is no fault of the kids in anyway. They seem like perfectly normal happy children who may or may not wonder why their uncle avoids them and acts distant compared to all their other guests who give them constant attention and fun.