What is it about Marriage?

What is it about marriage that changes everything in your life? Why is that before you say those words you are one person and after you are different? Why is it that we change and our relationship with our loved one(s) changes if only just a little bit, or a lot? Why do some fear it and some crave it?

That moment in which we say (literally or not) "I want to be there to witness the rest of our lives, and to partake in, share, and explore all of life's possibilities with you, as we so choose, for better or for worse." and we feel elation, happiness or for some - dread?

There is something magical an powerful in that moment where we say "I do." or "I will." that we know that nothing in our lives will ever be the same again. Why?