If I went through the mirror, Lucario would the one to follow. He would be a great dog! :D

Spike: “I am the weirdest boner right now.”

@Background Pony Number 17  
Much like his contortions here, Spike level of loyalty and care for her will forever be, I kid you not, a little frightening.

You know, you have to give Spike credit here.
 
Twilight Sparkle only went through the mirror because Celestia told her that it was the only way to retrieve her crown. We could see that she wasn’t really eager to go. It was probably only her trust in her mentor that gave her the courage to take the jump into the unknown.
 
Spike, on the other hand, watched Twilight get sucked into the portal, and immediately lept in after her. Didn’t even pause to think about it. Not even Rainbow Dash showed that level of loyalty.
 
It just struck me that this is why they turned Spike into a dog–at that moment, he truly was Twilight’s best friend.

“Ow, ow! I didn’t think inter-dimensional travel would involve muscle-pulling!”