more country than bluegrass, "you're my best friend" as sung by Don Williams, though it sort of jumps the gun if the couple dont already have children:
"You're My Best Friend"

You placed gold on my finger
You brought love like I've never known
You gave life to our children
And to me a reason to go on.

You're my bread when I'm hungry
You're my shelter from troubled winds
You're my anchor in life's ocean
But most of all you're my best friend.

When I need hope and inspiration
You're always strong when I'm tired and weak
I could search This whole world over
You'll still be everything that I need.

You're my bread when I'm hungry
You're my shelter from troubled winds
You're my anchor in life's ocean
But most of all you're my best friend.

You're my bread when I'm hungry
You're my shelter from troubled winds
You're my anchor in life's ocean
But most of all you're my best friend...

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I attended a wedding where the band played the Tennessee Waltz; thankfully no-one sang the lyrics.