FIRSTLY...
It's interesting if you look at your question.... "Why do people get accused of being either positive or negative?"
I would say people don't get accused of being positive. Think about that for a second the read on...
I would say people don't get accused of being negative either.
The thought-provoking word you've used in your question is 'accused'. This seems to imply there is something wrong with either being positive or negative. In fact, I believe there is nothing wrong with either.
SECONDLY...
If someone 'accused' you of being positive would it get up your nose? or would you feel like you've received a compliment?
At the very least, you are unlikely to be so injured as when someone 'accuses' you of being negative.
THIRDLY...
People often say something other than what they mean... with sometimes terrible consequences.
Compare these two comments
"You are stupid" ~ Implies something definite.
"You are being stupid" ~ Implies something indefinite.
Sadly, too many parents don't realise the difference.
So... an awareness of the use of language is useful.
FOURTHLY...
We all act differently in different situations or relationships. You will probably act differently when you are with your dad compared to when you are with your close friends. And different again when you with your boss/teacher.
The only person who knows all the versions of us is ourselves.
So I'd suggest taking it with a pinch of salt when someone puts you in a +/- box.
You know better.
LASTLY...
I want to share a poem with you. I would say your dad is right about the power of words. But too simplistic if you think that just saying someone is 'being positive' is enough to help them BE positive. Sometimes it takes a few more words to get a message.
If you think you are beaten, you are.
If you think you dare not, you don’t.
If you’d like to win, but think you can’t, it’s almost certain you won’t.
If you think you’ll lose, you’ve lost.
Throughout the world we find success begins with a person’s will.
It’s all in the state of mind.
If you think you’re outclassed, you are.
You’ve got to think high to rise.
You’ve got to be sure of yourself, before you can ever win a prize.
Life’s battles don’t always go to the stronger or faster one.
But sooner or later, the one who wins is the one who Thinks (s)he can.
Personally I think there's a lot of truth in the poem. But that's just my opinion. Others will think it's nonsense. And that's ok too. The difference will be in the actions I take from reading this compared to the person who thinks it's nonsense.
QED
PS. It is your choice to resent people who so freely use these terms and write people off this way. But when you are aware that IT IS your choice. Why choose resentment?