I get asked all the time how I learned as much as I have about jQuery. And then answer really consists of 2 things.
2) I do a lot of jQuery Projects.
There are 2 main purposes of jQuery.
2) Handle cross browser craziness.
At any rate, lets talk about a few cases where you would NOT want to use jQuery.
1) Extreme speed without caring about browser compatibility.
Its the same principal as, code will be faster in C than it is will be in PHP.
2) Element properties
This one I just found because I was doing something in jQuery I wasn't supposed to. This is actually correct behavior, but I really love this hack. What I'm about to talk about is in this video http://www.youtube.com/watch?v=8G_i615QLCg
So long story short. Its important to understand the core of something before you start using layers on top. But we can't expect too much, this is how the world is going now a days. We use computers on a daily basis but how many of us know how they actually work inside? As time goes on, we as people tend to ignore how things became how they are, and I think thats not a good thing. We should always learn from the past, respect it, and appreciate why things are the way they are now.