I don't see marriage as a necessity, but there is a tradition behind it and if done for the right reasons, marriage is a really nice idea.
I want to get married to my current partner because when we choose to have children, I would like our family unit to be traditional. I would like to have the same surname as my children and my partner. I want to provide my children with stability and a consistent set of parents who teach teamwork even through the hardest of times, and while this is possible without marriage, the concept of 'togetherness' is far more visible through a legal bond. I want the financial and legal benefits of a marriage - if one of us gets sick, visits to the hospital and any monetary issues are far more easily explained and sorted. I want my identity to be as that of a couple, because marriage means that I have entered a lifelong union, thereby changing my identity as an individual. I want to promise my partner that I will be with him 'forever' because I truly believe that spending my life with him is a privilege.
Of course some people may not agree with me, and they do not have to. But I do want to get married despite it becoming an increasingly outdated trend. I would never judge someone for not wanting to get married. The divorce rate is far too high because many people do not get married for the right reasons and do not respect the sanctity of marriage.
I think some people tend to expect marriage because we have been taught about its past existence in society. It isn't a rite of passage in the slightest, it is purely something that people want to do!