Well, you can't vow the impossible. You can't vow to love someone for a lifetime, as it's something you don't really have any control on.
It makes no sense.
Edit: and I agree with Jess
Staying married for life requires lots of works, but it is definitely NOT impossible! I've seen too many happy, lifelong marriages to belive otherwise. Marriage is for two people who love each other enough to want to stay together for life. It requires a mature decision, and is not something one do casually. That's why the vow is as follow:
The groom says:
I, N., take you, N., to be my wife.
I promise to be true to you in good times and in bad, in sickness and in health. I will love you and honor you all the days of my life.
The bride says:
I, N., take you, N., to be my husband. I promise to be true to you in good times and in bad, in sickness and in health. I will love you and honor you all the days of my life.
Or: "I, ________, take you ______, for my lawful wife/husband, to have and to hold, from this day forward,
for better, for worse, for richer, for poorer, in sickness and in health, until death do us part."
Rings:
N., take this ring
as a sign of my love and fidelity. In the name of the Father, and of the Son, and of the Holy Spirit.
Of course, you can't tell the future... but if you take your vow seriously, you'll try your best to work out the problems that may arise during the marriage.
The rate of divorce is high, because people don't want to work out the problems. They choose the easy way out rather than working hard at their marriage.