One moment I like in Secret of My Excess is when Spike was about to confess his crush to Rarity, she shut him up.
It’s almost like she didn’t want to outright say it wouldn’t happen.
I like to think she can relate. Rarity’s a dreamer. She dreamed she a commoner could actually marry a prince like it was a fairy tale.
Maybe Rarity wants to keep that sense of childlike wonder and dreams alive in him. Maybe just like how she was as a filly.
And besides, maybe it could happen even if only 1% chance if Spike grew older. But the importsnt thing is to keep the dream alive for his possibly last moment.