Why did they turn Holly into such a terrible person?

I mean, in season 1, she made some bad choices, but you could forgive her because she was 16. After that (seasons 3 and 4 especially), she was just terrible!
She was a bad friend to Tina (asking her and Gary to leave her own birthday weekend, so she could be with Vince, telling everyone that Tina had feelings for Gary), she was an overly paranoid girlfriend with Henry, Ben and Vince - feeling threatened every time another girl even looked at them, and she was horrible to Val a lot of the time (rescheduling her wedding shower without telling her so she could spend time with Ben).
I don't understand why the writer's had to make her character so unlikeable...it took away from the humour.

I'm not insane! My mother had me tested!