This is the Teen Wolf version of the Hemlock Grove story of the same name. If you're a Hemlock Grove fan and Romancek shipper, you should check out the other version of this story. There are small differences between the two. This is like with the Unquiet Hands epilogue where I couldn't decide which I liked more, so I just published both.
According to everyone else, they don't say it enough. In fact, they almost never say it. Sure, there's the occasional slip in a desperate situation (when everything seems to be falling apart and the only thing they're sure about, the only thing they can ever be sure about, is each other), or when the whole world is bearing down on them (when they're standing at the precipice of something terrifying and the calm before the storm is settling deep in their guts and they feel like they should say it, should put it out in the open just in case they don't get another chance). But, really, other than that, it doesn't come out.
That's a lie, though. Because they say it all the time. They're always saying it. Maybe not out loud, but neither of them have ever really been good with words (well, one of them just doesn't have a lot to say and the other just doesn't actually say much even though he talks all the time). They don't need to say it out loud. And just because people can't hear it, doesn't mean it's not there. The truth is, they say it more in a single day than most people do in an entire lifetime.
Because they say it in every touch. It's in the gentle hand resting in the small of a back for the briefest of moments as they pass each other in the tight hallway ("Why the hell is this hallway so small?! This place is huge!" "Maybe you're getting fat." "I wish you'd never found your long lost sense of humor."). It's in the countless, feather-light touches to the neck, the hair, the back of the hand. It's in every caress when everything is dark and they just need to be sure someone else is there; it's in every pinch and prod and push when they're lying in bed at night trying to make room for themselves without moving too far away from each other; it's in every single breath they gasp over each other's skin when there really are just no words to say.
It's difficult to pinpoint the exact moment they realized it. It's even harder to recall a time when they barely touched at all, except, as is often pointed out, when one of them would slam the other into wallsforChrist'ssake ("Don't forget the steering wheel that one time." "Yes! And the steering wheel!"). It's nearly impossible to find the point in time when the touches went from acknowledgements of the other's presence to silent secrets slipped to each other throughout the day. Neither spend too much time thinking about it, because that time has passed and this is where they are now and if they think about it for too long, they might miss something the other has said. They spend even less time trying to explain it. In fact, they don't, really. They never bothered. No one else would understand and, frankly, neither of them want anyone to try to understand. It would annoy the hell out of bother of them.
Scott doesn't believe him usually, Stiles thinks, whenever he stutters out the words in a hesitant sort of way that barely sounds convincing to him. He wants to tell him all about it sometimes, but he would only smile tightly and nod and still not believe him fully (they've spent too many years growing apart and into others that there's a disconnect and it hurts but it's there all the same and they just don't quite believe each other anymore). Derek doesn't have that problem. Of course he doesn't. Because Peter knew all along; Peter knew before any of them did. And he's probably the only person in the world who understands what they're saying every time they touch because he and Derek have been speaking that very same language their entire lives (Stiles is absolutely sure of this because he believes that beneath the nasty, sarcastic, douchebaggy, assholish, untrustworthy, smirkiness that is Peter he's actually got a huge soft spot). Still, Stiles doesn't envy Derek for that. While he would like his best friend (because even after everything that's still what they are) to see the truth in his words, he knows he wouldn't if he were in Scott's shoes. Because they've always had a lot of words for each other and even if they didn't say exactly those words, they could say words together. And when it mattered, they did say it out loud.
There would never come a time when he and Derek would ever need to speak the words in non-life-threatening situations. There would never come a time when they would want to. They've both been hurt too many times, and they've both felt the worst of what life has to offer. Their hearts can only hold so much. So when they need to let it out, they reach for each other and paint the words I love you into whatever skin they can find.