Chansey evolves from 220 friendship. Affection and friendship are not related at all, they are two completely separate things. Although Pokemon Amie can sometimes raise friendship. So the reason it hasn't evolved is that you haven't met the requirements for it to evolve yet.
Check this link: http://bulbapedia.bulbagarden.net/wiki/Friendship#Generation_VI for some ways to raise it's friendship so it can evolve.
Source: Serebii and the above Bulbapedia link.