The Hebrew bible treats the creation myth so that when God created the perfect being and found that it was lonely, he split it in half, forming the man and the woman. Thus marriage was the ultimate union for it symbolized the joining of the two halves; it represents perfection. It's thanks to several translations worth of changes and being adopted into a male dominated organized religion, that made "half" turn into "rib" and "companion" turn into "servant".
In Hindu tradition, Marriage is considered to be the apex of your life cycle; the holiest moment of your life, when you're most pure. It's pretty much all downhill from there, because the next recognizable milestone in your life is your death.
Point being, these did not become the social norms without a reason. We were literally bred to desire long lasting relationships. For you see, us humans did something that, from an evolutionary standpoint, would be considered incredibly counterproductive and started having offsprings that require multiple years worth of care before they're even semi-capable of fending for themselves. This requires allot of effort, in order to make sure that your seed continues into the next generation. As a result, babies of parents that were quick to abandon them, were less likely to survive and thus their characteristics were weeded out of the social gene pool. Fathers that stayed loyal to the woman that they impregnated saw their reward in form of descendants. Children raised in such environment, not only shared similar characteristics as their parents, but also remember what it was like to be raised in the care of two parents. As a result, they were likely to behave in the same way.
Fast forward hundreds, if not thousands of generations later and these behavioral tendencies are not only considered to be normal, but also the status quo that we should strive to maintain. This opens up the question of nature vs nurture, where we ask if we think in this way because our genetics demand that we seek out a soulmate, to love and cherish as the two of you raise your children, or if you were programmed by the social norms that you're exposed to, during your upbringing, in the forms of witnessing your friends and family members in stable relationships and fairytales/movies where the guy gets the girl and they live happily ever after.
Long story short. Love is complex. Don't expect an easy answer to this question.
