I have a tendency to gravitate towards mid-tier characters... that would explain my toonlink, zamus, and lucario. Only exceptions to this are snake and jigglypuff lol.
TeeVee, you aren't completelly right on that... peoples' mains are usually their mains for psychological reasons, and not for competitive reasons. Or at least, at first. Why do Ganon mains main Ganon? There's obviously no logical or practical reason, so you can safely assume a good 99% of the time that they simply like the character. Now then, this is where secondaries come in. Secondaries can also be psychological outcomes, but more often than not, they are when the player gets competitive, so to speak. They're only there for whatever their mains can't handle competitively. Hence, they are second to their mains. Your job can be bringing in money while your parents just love you; you prefer your parents over your job; this doesn't mean that your parents are always going to be their to give you a roof to sleep under.
Why would one main ROB? One could just as easily main MK if he wanted to win.
Why would one have MK as a secondary? MK is better that ROB, so it's illogical to have him as a secondary, a supplement to ROB, a worse character, who is your main.
I'm sorry for over analyzing that.