There are several reasons why your cat may hate grooming. Trying to groom out mats can pull on the hair and cause actual pain for your feline. There are cats who just hate being groomed.
I have three cats - one of whom will let you groom him until your arm falls off, one likes a good grooming; but, has a time limit of about 10 mins. and one hates it.
There are several things you can do to make the grooming experience more pleasant for both of you.
Try cutting out most of the mat before you groom. This will reduce the amount of effort needed to remove the mat.
Make your grooming sessions short - don't wait until your cat is desperate to end it. It all doesn't have to be done in one sitting. At the end of each session, do something your cat enjoys such as a play session or treats. You need to associate good thoughts with grooming in your cat's mind.
Also, check out the brush you are using with someone knowledgeable to make sure you are the correct brush type.