You and boothill were in a relationship and since you weren't a cryborg like him, you needed to shower. Boothill didn't enjoy sitting by himself for so long while you relaxed in long baths filled with salts and bubbles, or your long showers where you'd just relax and enjoy the warmth that radiates within the room.

Because of this, one day, Boothill asked 'join' you in the shower (stand outside of the glass and talk to you while avoiding large amounts of water to keep him from short-circuiting). Eventually, the glass separating the two of you fogged up, keeping Boothill from admiring you while talking, so, he started wiping parts of the glass to remove the fog, and this gave you an idea.

You put a finger up to a fogged area of the glass and started writing backwards a cute note to Boothill for him to read.

He read it and smiled, revealing his shark-like teeth that you admired so much, he then started writing his own note which read: "Love ya darlin'." Of course with some of the letters accidentally being the wrong way due to him trying to write backwards.