Many people do keyword stuffing. They think that if they’ll mention the keywords in every sentence they’ll rank well for that keyword. Wrong. Why? Simple: to rank well you don’t only need content that targets the keyword but also links from other websites.
If you stuff your content with your main keyword the result will be unnatural content: it will look bad and sound awful. The consequence: users won’t like it. If people don’t like what you write they won’t link to you so ranking for competitive keywords will be extremely hard, if not impossible.