Main parts of this puzzle are study, proper job , marriage, smart housing move and retirement plan! We shall Live like economist and study each step economically.
Is marriage really a requirement to life for most? Maybe for some but I still believe there are others that lives a more fulfilled life for themselves without getting married. There are also a few religious persons that there region or tradition forbids them from getting married or birthing a child.
Life, life is just as complicated as that. It's never fair to anyone nor does it present to some an advantage over the rest. Even the rich have got places that irritates them too.
We've just to keep trying and figruing ourselves out with every step, making mistake and correcting them.
It is! Life is incomplete without marriage or let's just say having a partner until you die. I know your point that they can still live to the fullest even though they don't have partners and even became successful, but trust me if you have everything money and you are getting old but still you don't have a partner to spend time on you will really feel a void, you feel incomplete. When I am still a teen I haven't thought about marriage or having a partner I am focusing on my career and earning money but if you already have enough money you'll start questioning your life and asking what is still missing? you can go to other places, eat in expensive restaurants and can afford expensive things then after you get home? you will feel lonely at that time.