Castiel's only just lost his purpose when the TARDIS appears meters in front of him, wood creaking in time with her signature vworp. It's silent for a moment and the lumber behind him collapses, splits from the pressure of the mournful fire, and the weight of the two bodies it's devouring. There's a sweet, sickly smell floating through the air which Castiel can only just detect over the all-encompassing ashes drifting down from the sky.
"A volcanic wasteland?"
Castiel didn't expect the Doctor to say anything, but it appears he doesn't know him as well as he should. He answers hoarsely,
"I thought it… appropriate." The eruption's decimated the entire island, leaving behind few survivors in the form of burrowing animals and insects. A slight alteration in pressure, a few of the wrong gases slipping through the wrong cracks, and lava'd rained down with the sole purpose of smothering the island. It's done its job very well.
Like the Winchesters had done their job, lived their life together, and eventually died together. Castiel doesn't know how. He hasn't needed to keep an eye on them in so long, it sometimes feels like they never knew him at all. The pounding in his sternum and the ache in his throat say otherwise.
The Winchesters. All-encompassing, destructive, impossible forces of nature who died like everything else has to.
"I didn't know you were so poetic."
Castiel turns to, finally, take a look at the Doctor. He's gazing up at the ashen clouds, hair half-covering one eye, right hand in his tweed pocket and left palm-up and collecting ashes like a child's would collect snow.
"What do you mean?"
"Well, an eruption like this seems like a tragedy from a human perspective, doesn't it? Like the Winchesters. But maybe, just maybe, the impossible human perspective is limited." The Doctor turned his gaze from the sky to Castiel, lips upturned but not quite smiling. "In a few decades this whole island will be lush and green again, and better than ever. It'll have exotic wildflowers and odd little creatures running amok and birds making a ruckus in the trees, and, boy, will those trees be beautifully strong. Like the Winchesters. Maybe a few will be as tall as Sam."
Castiel almost jumps when the Doctor's ash-filled hand claps his shoulder, rubbing it in for good measure. His muscles reluctantly relax as the Doctor's palm makes circular movements against his chest, only to clench up again when the Doctor takes a step closer.
"Just because they aren't alive doesn't mean they can't do good. For the world. For you. Cas, it may hurt now but I swear to you it will get better, easier, and one day you'll look back to your time with them and see how much they've changed you for the better. If the volcano's ashes are the foundation for those beautifully strong trees, imagine what our two boys can do for you."
"I…" Castiel swallows—a habit he's picked up from Dean—and looks the Doctor in the eye. "You said it takes decades."
"Of course it does." The Doctor winks and quirks a thumb towards the TARDIS. "That's what I've got a time machine for. Skips all the Tuesdays. And even if it lands on a Tuesday, there's always an adventure out there—people to save, things to do."
Castiel ducks his head, hyperaware of the blood rushing through Jimmy Novak's veins and the Doctor's still hand on his shoulder, no longer messaging. He doesn't want to leave Earth. He doesn't even want to leave the island. He wants to sit in front of Sam and Dean's joint pyre, wait until the flames go out, watch as their ashes and bones become one with the sediment and, eventually, as a bit of green sprouts from where he stands now. He wants to experience the years and the pain, and he doesn't want to deserve anything more. But he knows he does, because he sees it in the Doctor's emerald eyes (that perhaps remind him a bit of Dean's except impossibly gentle) and he thinks he understands exactly what the Doctor's offering.
Companionship. Support, love, and companionship, all wrapped up in the only man who could possibly help him.
"Yes."
The Doctor smiles, slips his hand from Castiel's shoulder to his wrist, and asks, as they leave the Winchesters' ashes behind, exactly where and when he wants to go first, and wouldn't a nice, quiet vacation planet be snazzy, and, Castiel, I can't tell if you're teasing me or you sincerely believe sitting in a lounge chair doing nothing for entire weekends is the proper way to relax.
"Take me to the future first," Castiel says, not quite leaning on the TARDIS console, eyes roving fondly over the Doctor's limbs fidgeting towards the controls. "I want to see those trees."