I'd like to add to what raincoaster posted. All themes except P2 are designed to "showcase" our post on the front page and that means comments are collapsed. When we click a post title or a leave a comment link we view the entire post and all comments on it's own page. In a new blog without many comments it may be hard to comprehend who this it. In an established blog with many comments if they were not collapsed on the front page it would be a mile long, and take a very long time to load. Page loading time is a page ranking factor. Studies show that visitors will wait only 3 - 4 seconds for pages to load. So in an established blog if comments were not collapsed on the front page we would lose visitors who click out rather than waiting for the page to load.