The answer to this question is a complex one that spans the fields of evolutionary psychology and behavioral ecology. It includes factors such as mate choice, sexual selection and the trade-off between quantity and quality of offspring.
It is also important to remember that the process of mating and mate choice are continuously evolving in response to the ever-changing needs of individuals and societies. This means that even if you and your partner are very happy with each other now, it is still important to take the time to check in every once in a while to see how your relationship is evolving over time.
While it is tempting to speculate on what the world would be like if humans behaved more like animals, it is important to remember that the adoption of animal-like behaviors could have serious consequences. For example, if human society grew accustomed to a strict pecking order, similar to those found in some ape societies, it could lead to increased discrimination and inequality, as individuals would be judged solely on their appearance and physical prowess.
Another thing to remember is that olfaction plays a very significant role in mate selection. Researchers have determined that certain gene complexes (such as the major histocompatibility complex, or MHC) play a critical role in scent recognition and mate valuation, creating a unique odor profile for each individual, much like a fingerprint. In addition, it is believed that pheromones and chemosignals are key in generating feelings of attraction and commitment.