The tradition of marriage was borne in the concept of carrying on the bloodline. Taking care of your parents as they age is noble, but then they’ll die. If there is no generation after yours, there will be no one to take care of you as you age, and the family bloodline and perhaps even the name, will die.
Pretty much all living things have the primal instinct to eat to live and live to procreate. Humans are the only ones who have sought to have additional “meaning” to our lives and to occupy our time with “careers” to earn “capital” in order to acquire “assets”.
Whether or not to marry, and whom to marry, are now very personal decisions that we have have the luxury to make as societies become more progressive. For some, it is the “next step” in the things they were taught that people “should do”, followed by having kids. Many do not pause to contemplate whether it is really what they want or if they are well-suited to become parents, and many are not mature enough to handle the pressures of having someone be 100% dependent on you and the to invest the energy in training them to become independent. Thus, a lot of families have dysfunctional dynamics, and the members of the household end up miserable. On the other hand are folks I know who would make awesome parents, but some have difficulty getting or staying pregnant. Those who choose not to have kids because they feel they do not want to make that commitment get called selfish – even in less conservative cultures.
I personally feel that marriage is a legal contract between two adults who choose to become each other’s family. It has advantages for people who will combine their assets and have rights to act on the other’s behalf in emergencies. It makes sense for most of those who plan to have children together. If there are no plans to have kids, or the partners can not have children, then it is more of a personal and financial choice. I’ve known people who were married that didn’t seem to share a devotion with each other, and I’ve known unmarried couples that were clearly committed to one another. Marriage and commitment can each exist without the other.