This is why some people stick together when the dopaminergic thrill is gone. In animals, the responsible chemicals are oxytocin and vasopressin. Interestingly, these calming chemicals are secreted by the same hypothalamus that fuels our lust.
Some may see the above systems as a kind of progression in a relationship. First lust (“hey, he or she is cute”), then romance (“I’ll write a love song”), then marriage (calmer and cozier). While it’s true that these aspects of our brains and our relationships change over time, it’s important to remember that they never dwindle to nothing and often interact in important ways. For example, oxytocin and vasopressin are connected with the dopamine reward system as well. Perhaps that’s why it’s a good idea to refresh the romance now and then, so affection can bloom.