Because a good person does not equate a positive person a lot of the time.
And if it didn't, it wouldn't ensure only pleasant experiences one's entire life. I reckon even a lot of advocates of positive thinking and law of attraction are fully aware that even being 100% positive 100% of the time won't bring only good to you ever.
And if they don't I think they're in denial about some huge parts of life. And actually that's the particular place where I'm very anti some new age schools of thought which focus on only
peace. Those ideas are very, very dualistic, and I think they're unhealthy. They're also a very good way to accidentally convince people that if bad comes to them, that it's their fault. (i.e. a not uncommon thing in new age seems to be people ending up believing if they become ill it's their fault
. Notions of blame are never helpful.)
All things have a shadow side. You acknowledge the good, the bad, the darkness and the light, and you're living in balance and seeing life as it is. Most spiritual systems that I resonate with eventually come down to the idea that all things are connected, i.e non-dualistic. I'm not keen on unnecessary pessimism, but I find denial equally unhelpful. Also, though I've used the terms myself I think terms like 'good' and 'bad' are essentially flawed. Most of us are relative mixtures of all sorts of different attributes, which tend to manifest in relation to our life experiences. Plus 'good' and 'bad' characteristics are hugely determined by a particular society, era, culture and as such are incredibly subjective.
Plus, grim as it may be, we have to experience death, which is probably most of our biggest fear within our own shadows. It's as much a part of our life cycles as life itself. One of the ways we cause even more huge amounts of pain and confusion in our modern Western world is by refusing to fully acknowledge death.