Castiel was always misplacing scraps of paper he wrote notes on.
So he had taken to writing important things on his arm.
With a few coloured biros in his trench coat pocket, he was always ready for whenever he needed to write something down.
It especially helped that he was an angel - his 'mojo' made the writing permanent, until he didn't need it any more.
Dean and Sam's cell phone numbers was the first permanent note he wrote.
—
When Cas fell, the permanent notes turned into tattoos. He needed the reminders more than ever - now that he had lost his grace - in order to even survive the basics of human life.
Dean had taken to writing notes on the blue-eyed man's arm - 'remember to eat breakfast' or 'don't forget to shower!' or even 'make sure you get dressed!' - whenever he had to leave the fallen angel at the bunker on his own.
After a few weeks, the messages started to get more domestic - 'sleep! you need it - you look like you haven't in days!', 'call me if you need help with anything, we won't be too far' and 'don't do too much reading while I'm gone!'. Charlie thought the messages were cute - Sam did too, but didn't want to say.
—
One day, when Dean was busy writing his reminders on the fallen angel's arm - he and Sam were going to a salt-and-burn not too far from the bunker (just slightly too far to come back at night) - when he unconsciously wrote 'I love you'.
Castiel didn't notice it until the brothers had been gone for a few hours, but, when he finally did read it, it brought a huge smile to his face.
Instead of staying in the Bunker like he said he would, he decided to make a quick trip to the nearby shopping centre. He was sure there was a tattoo parlour there.
There was.
—
When the brothers returned the next day, around lunch time, the blond was the first to notice the red surrounding the green words on the dark-haired man's arm.
Castiel had got the tattoo artist to go over the blond's words, forever etched into the slightly tanned skin.
When Dean saw what he had inscribed into his arm, he couldn't help but smile at his mistake.
"I see you took a short trip out while we were gone." He joked, brushing a thumb over the raw skin.
Instead of answering, Castiel took a light blue pen from his pocket, and uncovered the forearm on the hunter standing before him.
In his unmistakeable handwriting was written 'I love you, too'.
It didn't take too long to decide whether he was going to do the same as Cas, so he and the fallen angel made their way to the shop, and they both came out with matching tattoos.
They both loved them, and kept them for years to come.