Keyword stuffing is unethical practice of SEO. It may get your website spammed. Google may impose penalty or your website can be de-indexed by this practice. Keyword stuffing is not liked by crawler. It is like as you are imposing your keywords forcefully in your web page.
You know that you can use 125 characters in title. What if you consume all character limit by just putting your keyword again and again in title. For ex. Let’s take an example of keyword “Keyword Stuffing”.
<title>keyword stuffing, keyword stuffing…………………………….Keyword stuffing</title>
This is called keyword stuffing. It can make wrong effect and your website ranking can be down.