This glitch is rather easy to use and it benefits both your relationship with Karen and your dog's affection level.
To perform this glitch, simply take your dog to Karen and talk to her many times when you are holding it. It is best to do this when she is inside a building because it takes a while. As you are talking to Karen, you will see her heart level rise.
There are two side effects to this glitch. First, Karen will be at a red heart (or where ever you stop). To marry her, all you need is the kitchen and a blue feather. Also, your dog will like you more. If you feed him and pick him up and run him every day, he might be able to win the first Winter Dog Race.