Phil Coulson was the Handler of Strike Team Delta for 11 years.
He knew that the minute Clint decided to bring Black Widow in as an asset that this would be an everlasting partnership.
He just didn't know how deep until later.
When Natasha first came to Shield she had a hard time trusting people except Clint.
He knew that their feelings ran deeper than they let on when Clint took a bullet for his partner, and Natasha refused to leave his side until he woke up.
Phil also knew when their relationship jumped to the next level.
And despite Shield's fraternization rules he didn't stop them.
They needed each other.
They only have each other.
Two Halves of a whole.
One incomplete without the other.