I always thought itches and tickles were our brains trying to tell us their is something crawling on us. This goes back to primal times however. When we slept on the floor or in trees with spiders and ants. One reason for tickles/itches could be our brain thinking something is on our skin when it is not. This could be why scratching that itch is so satisfying,...
http://www.bbc.co.uk/programmes/b07dx75g/episodes/downloads has all the available downloads of a Radio 4 series answering various science based questions. One of them, from June this year, is specifically about itching and scratching. There is a bit of light hearted stuff in the podcast at the beginning and at the end, but I think you'll find the answer to both your questions in that programme.
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