I have seen many websites Google crawling regularly. Why Google bots takes so much time time (6-8 days or so) to crawl? What should I do on my website so that it crawls regularly?

You need lot of content to get crawled more often. 6-8 days is normal. I have a site with more then 50K pages, gets crawled in that rate. So you can see its normal.

Sites like social networking sites adds thousands of pages everyday. Even on news websites like cnn.com content changes daily. SE knows that. Only sites like that get crawled at a faster interval.