Think of an older dog you know, one that may have been with you for many years and might not, at this time, be the total focus of your attention. If you do not have or know of such a dog, a human friend is a perfect stand in for this exercise.
Think back on a time, perhaps long ago, when first you met this dog. Allow yourself to really remember what it was like, then remember details about the meeting, what time of day it was, where it was, what you were wearing and so on. Now, remember the moment when you first fell in love and also then, remember how you felt at that time.
Holding the feeling steady, allow time in your mind to pass as it does and consider the dog as it is today. Reflect on how your feelings towards the dog have changed over time and then as a result of this exercise.