After the hole thing with the Tree of Harmony, Spike, the future, the Great War, the Megaspells, and the Last Day, Twilight could not focus on anything aside from Future Spike's warning. That night Twilight couldn't sleep, she kept having nightmares.
The next morning, Twilight spent the whole time modifying the mirror portal. "Twilight I really think this is a bad Idea." Spike said cautiously. "You've already messed with time travel more than once."
"First Spike we've only been to the past never the future and if I can figure out what starts the war I can prevent it." she answered.
"But we already know that it's coming, if we know that can't we just keep an eye out?" Spike asked.
"That's not good enough Spike. And this is the only way." Twilight said.
"But what if something goes wrong? You just became the new ruler, and if something does go wrong and the war still happens we'll need you." said Spike.
"Don't worry, I'll be back." she said before turning and walking through the portal.