Why Do I Hate My Step Mum?

Well, to be honest, there are lots of reasons, even if she is a nice loving  person.
You've not mentioned what kind of person she is, though I had a problem with mine and still do.
I love my Dad and would do anything for him and don't think she's good enough for him.
You may dis-like her for the simplest of reasons, you think she's taking your Dad away from you, maybe he's spending more time with her that he used to spend with you.
Maybe she's taking command over the kitchen/living room/remote control and it annoys you as it's the house where you grew up, you may feel like it's your right to chose what to watch, what to do etc.
If your Dad is happy, there's not a lot you can do and not upset him at the same time.
Some people simply don't get along.
I'm not sure of your age or anything like that, so can't really advise on what action you might be able to take.

Because she isnt ur real mom, i have a step mom 2 and i hate her cuz the idea of my dad replacing my real mom with someone else doesn't make me feel good, thats probably the reason u hate her like i do