I think it can be different for different partners and situations.
For a new partner, there's the thrill of discovery, of seeing their body, finding out what turns them on, them finding out what turns you on. There could even be something completely new if the partner introduces something you have not experienced before, e.g. you've previously limited yourself to straight sex and you're trying a same sex partner for the first time.
Having regular sex with a longtime partner like a spouse, there's a thrill in sharing each other's pleasure and body, in the familiar acts that express your feelings for each other. You can still learn and try new things, of course, but the thrill comes from that moment when your eyes meet as you cum in/on them, not any novelty.
Experimenting with that same longtime partner (trying something radical in the bedroom, adding another partner or couple) can then let you experience novelty while still also having that thrill of knowing each other intimately. Kind of hits on both of the previous points.
But I don't think you need "thrills" or experimentation to have an interesting sex life with a longtime partner, just an interest in knowing and loving them deeply and intimately. My own sex life with my partner didn't die for lack of experimentation, we just kind of lost that spark plus health issues limited her interest.