I tend to collect from shows I've enjoyed or from series where I love the art. Down deep I'm an art collector at heart, and I freely admit that I still haven't every series I have cels from. I keep the cels because I love them for their sheer artistic merit.
Of course, the shows that have touched me tend to occupy much more space in my cel books.
And like many here, I have the collecting gene. Bad. Cels are not my only collection.