I didn't think so until I adopted two black lab mixes. Just like kids, you think you can never love the next one as much but then you do and they have a different and special place in your heart that you wouldn't trade for anything. I will never forget my childhood dog 'Spooky' and she will always be in my heart but I know she is happy that I let two other dogs in my life to make me happy.
Yes they can. You always have wonderful memories of pets that you have had in the past. But when you think about them all, they all have their own unique personalities, abilites, likes/dislikes and smart/fun things that they do. I don't compare all the dogs I've had as to who was the best, I love them all for the unique animals that they were and are.