Sorry to be blunt OP but there WILL be a reason. There will be something about you that is un likeable.
I knew a few women like this, that are happy, smiley, want to be liked, and probably seem nice in their head, but are annoying and I would avoid in a social setting.
You would have to describe a bit more about yourself, but the things that happen to be why some people aren't liked much are:
Laugh too much at their own jokes
Too touchy feely and gives hugs and over the top gestures when she arrives
Negative moaners
Talk about yourself too much
Talk about yourself for ages without someone asking
Take up too much of the conversation
Im only saying this as I went to a wedding recently and there is someone there there is generally one of those women. If she catches you in conversation, that's it, you have 15/20 minutes of a run down of her recent holiday, venture, job whatever. You don't even have to ask she will just talk and talk and has such poor social skills that she doesn't realise the other person hasn't even asked or said anything in response.
But she would definitely describe herself as a happy, nice person and doesn't get why people don't really like her.