Spike- but you've seen the sizes of other dragons, what if I grow to their size?
Twilight- Spike, you remember what Zecora told us, dragons only grow in size with their greed, and you've got yours under control. And even if you do get huge, we'd find a way to accomodate you.
Luna- of course. And who would be better to protect Equestria than a large fire breathing dragon? Even at the size you are now, you've done great things.
Twilight- Luna's right, without you, who knows what would've happened? You saved the Crystal Empire, twice, you've been at our side thru all the hard times, and we'd be nothing without you!
Spike- hold on a second, Twi...
*Spike pinches her*
Twilight- OW! Spike! What the heck?
Spike- Gah, just checking to make sure you're real! *ducks and covers* I'm sorry!
Twilight- *shocked* woah, Spike, how freaky was this dream?
Spike- when the only sister you've ever known casts you aside, you'll know how I feel. I know I've done a lot of things for you guys, but I have to accept the inevitable, one of these days, I'll just have to leave.
Luna- young nephew, you make yourself less important than you know. There are some thing I must show you...
*Luna touches her horn to Spike's forhead and takes him back in time thru memories...*
I think sometime in the future Spike will confront his fears!
I think it would be good development for him!
You know, I think he already has.
In Spike at Your Service, the little dude feels he owes a life-long debt to AJ, and resolves to himself that he has to tell Twilight. When Twilight doesn't react, Spike is absolutely heart-broken, though he soon commits himself to serving Applejack instead.
Even though this episode was only played up for laughs, I like to think of it this way. Spike faced his worst fear to keep a promise to a friend.