Turns out, George Lucas initially hated the line and he apparently went apeshit over it. Lucas originally had Han Solo say "I love you, too" in the script. But most of us know that Harrison Ford came up with the "I know" line on the spot.
In a 2010 Vanity Fair interview, director Irvin Kershner explained that the "I love you, too" line "just didn’t seem right for the character of Han Solo," and Ford agreed so they started exploring different reactions that Han Solo might have. Kershner explained:
"We were into the lunch break and I said to Harrison try it again and just do whatever comes to mind.