Why feel negative 50% of the time?

Love this stuff – signed up for SCS to start in August, and have delved into the work already. I have a question that may be purely semantics – but maybe I’m not understanding some underlying theory:
If we can create our feelings through thoughts, then why accept that we should feel negative half of the time? I understand that there are times when we want to feel negative (loved one dies, friend gets hurt, etc.), but these circumstances where we WANT to feel negative probably don’t make up 50% of our lives, right?
So are we doing models to strive to feel “better” or positively just 50% of the time? and accepting negativeness the other half? It seems to me like this model work is looking to replace negative feelings with positive ones, but maybe we shouldn’t (or it’s a losing battle) if our brains are programmed to thing negatively half the time.