My first Canon (AE-1 program) was a gift from my dad. He was a Pentax shooter, and like most dads, probably wanted better for his children
In hindsight, it seems like a ill conceived idea as it would have made more sense for everyone to be using the same mount. But at the time, brand snobbery was very big. My dad always thought his competition entries were at a disadvantage when he listed a Pentax camera with Sigma lens.
That was a long time ago, and I've since owned a number of different systems. Throughout most of the 90's, I predominantly used a Mamiya RB67 kit. Recently, I've started playing with an Olympus m43 camera. I've also owned Yashicas and Minoltas. I'm constantly wondering if I should switch to Nikon, Sony or [insert brand of the month here]. But no matter how far or how wide I roam, I still call Canon home. Canon and Nikon are really the only two companies with the complete package. Once you've bought into one system, there is rarely a compelling reason to switch sides.