Grantaire doesn't know when it started. He was hesitant to join the relationship with Enjolras and Combeferre because he loved Enjolras too much, had no romantic feelings for Combeferre, didn't want to share Enjolras with him. He doesn't know when he fell in love with Combeferre.
Maybe it was his perfectly-timed sarcastic remarks which caught Grantaire off guard and made him smile, maybe it was the jokes he made every so often that would end in Grantaire laughing so hard he couldn't breathe. Maybe it was the way he blushes under his glasses when complimented, or how his whole face lights up when he smiles, or the way Grantaire fits under his chin when he holds him tightly and makes him know he is loved. It might have been how he kisses Grantaire, fervid but not messy, or how he gasps when Grantaire sucks a bruise into his neck and gets him off with a steady hand, or how he whimpers and moans when Grantaire won't let him come, or how on the rare occasion he tops, he'll slam into Grantaire with passion and say how much he loves him until Grantaire cries.
There is one moment that stands above the rest, though, in the saga of how he falls completely and utterly in love with Combeferre. Grantaire is bored and alone in his apartment, Jehan is somewhere else, so he decides to drop some acid because its been a while and he has nothing better to do. He takes a hit and for the first few minutes is okay, but then everything in his vision starts swimming and his head feels like it's going to break apart. He realizes, with utter panic, that he's having a bad trip, and he's seeing double and is having trouble breathing, and he buries his head in his hands to try to relieve the pain but it doesn't help, and he's beginning to panic, and needs someone to help him before something even worse happens. He pulls out his phone, struggling to unlock it and get to the contact he wants- Combeferre. He's a doctor, at least studying to be one, and also less likely than Enjolras to berate him for taking acid in the first place, and his almost-constant calm demeanor is just what Grantaire needs right right now. He taps on the number with a shaking finger and holds the phone up to his ear, trying to breathe correctly.
"Grantaire?" says Combeferre when he picks up, "Hey, what's-"
"Combeferre," Grantaire groans, voice trembling, "I'm having a b-bad trip and I need, I can't, everything is spinning and I'm freaking the fuck out and-" He breaks off mid-sentence because he can't breathe.
"Oh, Grantaire," Combeferre breathes sympathetically, "I'll be right there, okay? Hold on for me, and stay still to make sure you don't hurt yourself. You have to breathe, Grantaire, and try to stop panicking. I know it's hard, but I'll be right there and use the key you gave me to let myself in. Hold on, I'll be right there, I promise."
Grantaire puts the phone down and takes a deep breath, which doesn't help. His vision and head are still fucked up and he just closes his eyes, tries to calm himself down, waits for Combeferre to come help him.
Combeferre grabs Grantaire's key from the bowl by the door and races out the door, sprints from his apartment to Grantaire's. He comes in the apartment door out of breath, and immediately finds Grantaire, curled up in a ball on his bed. Combeferre rushes over to him and pulls him up.
"'Ferre?" Grantaire asks, obviously disoriented. His eyes are dilated, his face is pale and he's sweating profusely; Combeferre immediately wraps his arms around his boyfriend's shaking frame. Grantaire lets out a sob and buries his head in Combeferre's chest, his own chest heaving, and desperately grasps the fabric of Combeferre’s shirt.
"Hey, I'm here," Combeferre says, running his fingers through Grantaire's hair comfortingly, "You're safe, okay?"
"N-no, I'm gonna die," groans Grantaire, "I can h-hardly breathe and I can't... My head, Combeferre, it's going to explode and I'm going to die. I saw dark figures earlier, they're gonna take me, I swear-"
"Look at me," says Combeferre, gently tipping up Grantaire's chin, "Do you honestly think I'm going to let anything happen to you?” His voice wavers a bit, and he hopes that he sounds sincere enough. “You're safe. Trust me. You're going to be okay."
"I trust you more than anyone, but nothing can stop death, not even you, my world is splitting apart and I'm gonna die," Grantaire says, his eyes distant and shining with unshed tears. He fiercely hugs Combeferre again, settling in his lap, and Combeferre kisses the top of his head and rubs his back.
"You're safe, Grantaire, you’re not going to die. I'm not going to let anyone or anything hurt you."
"For the first time in my life," he says, then stops to laugh, "This is gonna sound so cliche, oh my god." He then stops laughing and continues somberly, "I feel scared. Really really scared."
"Again, you're safe. I won't let anything happen to you. I promise."
Grantaire shudders and holds even tighter to Combeferre, who's grounding him. "Do you love me?" he whispers.
Combeferre pushes him up and looks him straight in the eyes, tightly holding his shoulders. "More than anything," he says with utmost sincerity, cupping his face softly, "I love you, Grantaire, I love you and am going to keep you safe. I'm here, okay? The paranoia and the headaches and the dark figures won't hurt you."
Grantaire lets out a soft whimper and lurches forward to kiss Combeferre; it's a sweet kiss, but Grantaire has to break off quickly because he can't breathe. "There's two of you," he breathes, terrified.
"There's just one of me, sweetheart," says Combeferre, trying not to sound scared that his boyfriend is seeing double, "You're just hallucinating. You're going to be okay. How about you close your eyes and take a nap? You won't see double anymore. I'll keep you safe, okay? I've got you."
Grantaire kisses Combeferre again, greedily threading his fingers through his hair and ravishing Combeferre's mouth with his tongue, he's so scared, so in love, until he has to breathe and Combeferre pulls him close again and Grantaire rests his head on Combeferre's chest, breathing heavily. Combeferre pulls them so they're lying down, Combeferre's back against the back of the couch and Grantaire snuggled up tightly against his front. They stay like this for a while, Grantaire's breathing slowly returning back to it's steady normal rate.
"You shouldn't have taken the acid," Combeferre says eventually, "I'm not lecturing you, but from a medical standpoint it... wasn't a good idea. As you know. It's especially bad considering you now feel like shit."
"I know," says Grantaire into Combeferre's chest. "But I'm not exactly one to adhere to medical standards," he adds with a soft laugh.
"I'm aware," says Combeferre softly, "I know I can't change you, but that doesn't mean I can't try." Grantaire nods and settles even closer to his boyfriend.
"Things are still spinning but I think I'm going to be okay," he says.
"Good," says Combeferre, kissing his forehead.
"I didn't tell you this before," Grantaire continues, "When I asked you about it. But I love you too. A lot." He smiles and Combeferre grins back and kisses him sound on the mouth, grasping his hand and keeping him grounded, as he always will.