Just be honest! a friend can become a girlfriend in due course of time... that's quiet possible.
You just have to convince the officer that you will be leaving the country after your mentioned stay, take all necessary docs like bank statements, property papers, maybe even a letter from your employer granting you leave to visit canada.
During my first visit I wasn't even asked of any of those documents...I just had to show my travel itinerary that showed that I had a return ticket booked. Though that's not to say I will be traveling unprepared.
Also, I've been dating my girlfriend for over a year...so with that in consideration, I can't use the reason that one of my friends became my girlfriend during my first visit, because that would put me in a tight spot if they were to ask proof of my relationship with my girlfriend...