I believe I could be very happy living forever if I was healthy, safe and financially set. There are endless new experiences to have--hike a trail, read a book, try a food--when you're done with doing everything once in about a million years, start over.
I think a big part of why we lose interest is aging. Certain things are not as exciting, and they get difficult because it becomes harder to learn new complex things, or physical limitations grow.
For example, I used to think going to a club on Saturday night was the most exciting thing on the planet. Once I did that a lot and wasn't single anymore, it wasn't as exciting. If I had the physical ability to dance all night like I did back then though, and the $ to waste ;), I'd do it even now.