This is like the Nature vs Nurture theory. Personally, I think its a combination of both nature AND nurture. All cats have their individual personality traits just like us, and of course they also learn the facts of life as they are growing up. I think your cats are all so gorgeous because they were all brought up right, given a lot of affection and because they are all very nice cats as well.
I have a pair of littermates, now almost 2 years old. They arrived here at 5 weeks of age, were brought up the same, fed the same, handled the same, and loved the same, but there are differences in their personalities. Theo is very confident, flirty, very affectionate and boisterous, whilst Mac is gentler, quieter and perfers the other cats company to mine.
I think we can somewhat shape our pets, though. For example, if you socialise your puppy, your going to get a confident dog. If you bring a kitten into a stressful chaotic household, chances are you'll get a nervous cat. But there are exceptions to the rule. Some cats (and dogs) who have been brought up perfectly are still very nervous, whilst some pets who have been so badly abused by humans somehow remain friendly and affectionate.