Marriage or what to expect

Marriage..

What is the hidden meaning lying behind it? What is the point of spending your life with one person? What drives people to the marriage?

With the whole world settling around me, I start to think about the reasons why they are so afraid of being alone. Are people simply frightened to stay alone for the rest of the life? Or there are some other reasons like social expectations involved?

I am not 100% sure if a 20 year old can be looking for an eternity with someone they've just met. 

Moreover I dont think that any of them would care so much about the social expectations and standards which are usually more frightening for 30 year olds..

Then what drives us young and fresh in our 20 s to follow the road of the eternity?

Do we marry for all the wrong reasons or for true love?