A lifetime is a long time, and the little dragon knew (albeit dimly) that the twelve or so years he’d spent with her was nothing compared to the thousands that lay ahead. But in the rare moments that he let himself think about the future, he couldn’t for a single one picture a life without her in it. She had laughed when he told her so, remarked she should enjoy that while it lasted.
‘While it lasted.’
It ate at him. The future, the uncertainty, the thought of losing her comfort, her love.
He clung tighter.
Spike and Twilight
Either spike and twilight or spike and Rarity
Do to the extended life span of an Alicorn, Spike is happy to be able to stay by Twilight's side for as long as he can
Spike-Celestia, because 'she should enjoy it while it lasted' -> Immortal Alicorn
Spike & Twilight