Why do my cats not like each other?

Question
I have one cat who is nearly two (will be in july) and we recently (in Jan) got a new kitten who was born in early november. We have put them together since and have had a few problems in getting them to get along. Sometimes they will be fine in each others company but it seems to be making the older cat very unhappy to the extent that he has become unhappy with me and growls and sometimes hisses when i stroke him.
Why is this ?
How can i make relations better between both cats and my self.
Rachael  

Answer
At this point, just giving them time is probably the best idea.   You could separate them and start the introduction all over again, but do it very slowly.  I think you are probably past the point of that being of any benefit, though.

Cats can take months before they'll tolerate each other.  Sometimes they just learn to keep their distance from one another.  I would provide them each with their own things so they don't have to interact if they don't want to - own litter boxes, own bed, own toys, own food and water.

If the grouchiness continues much longer, I would recommend a vet visit.  Pain like an ear ache, a bad tooth or a belly ache can cause a cat to get awfully grumpy.