I think Best friend can be the best life partner.

May not be true in cases where one has different preferences as to few specific aspects of their partner. There are different type of expectations from a friend and from spouse.

It's certainly a great start

This is my heart's desire, to find my best friend who is also my lover. I long for this.

Truth.

I think so, too. That's what I'm looking for.

but not necessarily the best lover ..