Kurt’s been gone to NYADA for three weeks. They skype everyday. They text each other every hour. They call at least twice a day. They’ve tried to be as intimate as possible via skype. But nothing’s the same as it used to be. And all Blaine has left is to remember.
He lays in bed awake at night, long after the rest of the Anderson household has drifted off, just hanging on to memories of Kurt. He wraps his arms around himself and imagines he can feel Kurt in his arms. He closes his eyes and remembers the warmth that Kurt always gave off, even though his porcelain skin would suggest a slight coolness, so he lays and absorbs the phantom heat. He remembers lying with Kurt, so still that he could hear his heartbeat, so he lays listens to the magical tune of a phantom heart beat. He breathes in deeply and remembers the way Kurt managed to smell like all the most wonderful things in the world, vanilla and freshly washed sheets, flowers, hair gel, cinnamon, so he lays there and imagines he can smell Kurt there in his arms. He remembers the taste of Kurt on his tongue, always so sweet as they kissed, so he lays there trying to conjure up the phantom taste.
At night though, when he’s not trying to conjure up a phantom Kurt in his arms, Blaine is either actively missing Kurt, or actively trying to ignore that he misses Kurt. He makes up for all the duets he used to sing with Kurt in the car by doing it with Tina instead. Instead of walking Kurt to class he wheels Artie around instead. And instead of looking to Kurt to roll his eyes at something Mr. Schu said, he looks to Tina. Instead of discussing Algebra with Kurt, he does it with Artie. And that is him actively ignoring how much he misses Kurt.
The alternative to that though is the constantly worn and beaten look that he always has plastered on his face. Whenever he goes to get coffee, he always almost orders a grande nonfat mocha. Then flinches because Kurt isn’t there to drink it. Whenever he buys a Vanity Fair magazine, he always looks over his shoulder to gush. Then flinches because Kurt isn’t there to gush over it. Whenever he sports a new bow tie, he always gets excited to hear Kurt’s opinion. Then flinches because Kurt is no longer there to give an opinion.
When Blaine’s at his absolute worst though, is when he’s in front of a computer, mindlessly surfing, while actively ignoring that he misses Kurt, without trying to fill the void. He spends the whole time, subconsciously trying to answer a question. Why? He wants to know why he misses Kurt so goddamn much. He knows that he loves him. And when Kurt left, he knew that he was going to miss him a whole goddamn lot. He just didn’t know it would hurt so bad. So he whittles away his time. Absently seeing things, then forgetting them a minute later.
One day though, he finds his answer.
Why do you miss a person?
Because you became used to the idea of having that person around.
After that he spends a lot of time just missing Kurt. Missing all the things that having him around brought along. He misses the melodic sound of Kurt’s voice, even if he wakes up to it and falls asleep to it every night. He misses the way he used to hear in the halls, over a crowd of people. He misses feeling Kurt’s hand in his. He misses the way his tan skin fit perfectly into Kurt’s porcelain palm. He misses feeling Kurt’s lips on his. He misses the way their tongues swirled around each other. He misses the complicated buttons of all of Kurt’s ensembles. He misses how they used to laugh themselves silly when a moment of heated passion ended with Blaine’s fingers tangled uselessly in Kurt’s thousand buttons. He misses the way he could lay his head on Kurt’s shoulder. He misses the way he could smell, feel, and taste all of Kurt, just in that one simple motion.
He misses going to Breadsticks with Kurt. He misses the way he snapped at the waiters, in hindsight it was really so cute. He misses going to Between The Sheets with Kurt. He misses how they could spend hours in there, pouring over sheet music, looking for the perfect song for their Glee assignment. He misses going to the mall with Kurt. He misses the way every time they walked by a Gap, Kurt would jab Blaine in the sides and tease him mercilessly about Jeremiah. He misses going into Kurt’s room with Kurt. He misses how they could do nothing but stare at eachother the whole time, and just be perfectly content. He misses how just one look at each other could send their clothes to a heap on the floor, and their limbs into a tangled mess.
He misses that look Kurt would give him from across the room, that would make his toes curl. He misses the gleam Kurt would get in his eye when he talked about his mom. He misses the way Kurt could always pick a topic of discussion that would be a source of blushing for Blaine. He misses the way Kurt always managed to be so composed, even in the most uncomfortable conditions. He misses the way Kurt always looks at his toes whenever someone complimented him. He misses the way Kurt’s adam’s apple twitched when they talked about sex while they weren’t actively having sex.

Let’s face it. He just misses Kurt.
He misses Kurt’s idiosyncrasies.
He misses Kurt’s clothes, and their annoying buttons.
He misses Kurt’s voice.
He misses Kurt’s smell.
He misses Kurt’s taste.
He misses Kurt’s heartbeat.
He misses Kurt’s touch.
He misses Kurt’s everything.

That’s why he wrote Kurt this letter. To tell him everything he misses about him.