I just love lifting. There's nothing like it. There are many ways how it works. At first, during training, it feels great, must be endorphins and so. The training is also good for my mental side as well, I often have nice social interactions there, I get to do something totally different from my otherwise sedentary life, I get to spend time alone and I also get in a zen-like state while training, so it's a way of meditation for me. After training my body just feels better. On a longer scale, it's healthy for me. The improved strength also helps me in the physical challenges of my normal life, like cycling or yardwork or whatever.
I think the most important part for me personally is how I love lifting while training. It's something to get addicted to.
I do also have training goals that make my training more effective for reaching those goals. But the goals are just for the lifting.
TLDR: Pure enjoyment of the thing itself. "Muscle joy"?