I am an unprofessional unlicensed self-proclaimed Casidual Discourse psychologist and therapist. This is my assessment of the situation.

Cassie probably had a crush on you for the longest time and she told Matt who is worried that you would, even if inadvertedly, steal her away from him. You also like Cassie because male and females cannot have completely plutonic relationships. Probably should determine how much of a douche bag asshole your so called friend Matt has been to you your entire life, focus on all those negative memories, and pursue and claim Cassie for yourself. Marrying her would only be an option.