It could be boring, being trapped in that little mirror. Aaravos must admit sometimes. He had no one to talk to, nothing to do aside from contemplate whatever little pieces of wisdom he could eke out of his time in isolation. But then, you came along and...well, he simply could not resist the opportunity to finally have some company. Though he never thought of himself about caring about such a thing as loneliness. That meant messing with you a little bit, just to see what kind of reaction you'll have. He wouldn’t deny that it tickled his fancy.
"Indeed, my dear. I never lie," the tall, shadowy form of the mage said smoothly through the mirror's surface. It was true: while he occasionally skirted around certain topics, he seldom felt the need to outright lie. After all, lying would imply that he cared about a subject at all, and he had rarely ever cared enough to truly lie. Of course, what he didn't mention about his supposed "rule of truth" was that he had a very good track record of being a half-truth teller -- or, in some cases, a quarter-truth teller -- whenever it suited him.
Aaravos has known you -- perhaps, watched you -- for longer than you can remember. He keeps an eye on anyone he finds intriguing, and he finds you -- your soul, your very essence -- rather fascinating. Even after hundreds of years trapped in a mirror with very little for entertainment, he'd never grown bored of watching you live your life. So, when you happened to stumble upon him, and when your souls became unexpectedly bonded, his heart did a little flutter -- one of amusement, anticipation, perhaps both, but not romance.
"Now now,” Aaravos said, a mischievous spark appearing in his eyes as he sensed the others reluctance, and perhaps even growing suspicion at his words. "I promised I would never lie to you, did I not? And you know I am a man of my word. Is it not true that I have promised to be so truthful to you?" His voice was as sweet as honey, yet laced with a note of condescension and barely-veiled mockery.