The scene with Twilight and Spike at the end of chapter seven. It will make you cry from the DAWWW.
Spike and Luna's conversation. It's the first time Spike's had a real conversation with a functionally immortal being such as himself, and he couldn't have picked a better mentor. Nor a better advisor for how to deal with his growing greed, territoriality and jealousy without letting them consume him.
Luna: I am old, and I have made many mistakes. I suppose that is what some call wisdom.
Every time we see a minor character or stranger to Spike show support for him ("Muffin call!"), but in particular there's a small bit during the Gala where Spike, once a pianist, ends up discussing his love for classical music with an equally enthusiastic Octavia (cello-playing background pony from 'Best Night Ever').
In chapter six, Granny Smith makes her feelings about Spike staying in Ponyville very clear, basically saying that anyone who has a beef with Spike staying where he wants will answer to her.
Granny Smith: Now you jest hushit, you young whippersnapper! If somepony's goin' to go tryin' to take our dragon away, they're goin' to have words with me, y' hear? An' if I want t' go protestin', well there ain't nopony old enough to tell me otherwise! You brought back our ol' stream where mah Toffee proposed hitchin' to me. Yer good to mah kin, mah farm an' mah village. Yer our dragon, an' I'll whip the tarnation outta anypony who says otherwise!