There was a Spike story I read once, and am trying to find again. I believe it was a Sparity romance, but I'm not 100% sure on that.

In it, Spike and Rarity have the following conversation (paraphrased from memory):

Rarity: But Spike, don't you have any ambitions beyond just being Twilight's assistant?
Spike: Do you know how long dragons live Rarity?
Rarity: Well no, not really.
Spike: Celestia told me we can live thousands of years. If I'm very lucky, I'll get to spend a hundred years with Twilight. I plan to enjoy it while I can.

If anyone else remembers it and can point me to it, I'd be most grateful.