I am very happily remarried, and I do have faith that my marriage will last "until death do us part".
BUT, unlike my first marriage, I don't take it for granted that I am guaranteed that. I was very naive in thinking that, no matter what, my first marriage would last (because that was MY perspective). I think it makes me a better spouse - in some ways, my second husband is reaping the rewards of my failed marriage!
I don't think of it as a worry or cynicism, more like "eyes wide open" that pps have referred to. It also helps me that I am my husband's first marriage, and he was 36 when we married. He tells me he waited until he got it right, and that he had given up on "needing" to be married, so he's with me because he wants to spend the rest of his life with me.
I choose to believe him.