Always make affiliate links nofollow. Google’s goal is to provide its users with the best possible search results to their search queries. One of the main ways they figure out which webpages are the best is through links. The Google bots crawl the web, reading text. When they come to a link, it’s like an open door. They follow the link (walk through the door) and explore the site you linked to.
Websites consisting mostly of affiliate links have previously held a negative reputation for underdelivering quality content. In 2005 there were active changes made by Google, where certain websites were labeled as "thin affiliates". Such websites were either removed from Google's index or were relocated within the results page (i.e., moved from the top-most results to a lower position). To avoid this categorization, affiliate marketer webmasters must create quality content on their websites that distinguishes their work from the work of spammers or banner farms, which only contain links leading to merchant sites.