Did you know that when you are planning this life you make contracts with other people AND animals? One of the most fascinating things I've learned in the last year was this fact.
Why would you make a contract with someone? Our souls, along with our Guides and Masters plan what the soul wishes to work on in this lifetime. Sometimes this is called your primary lesson, but there are always many lessons we can learn in a lifetime. If, for example you wanted to work on learning to love more, you might ask someone who has a lesson that might resonate with yours, to help you. This doesn't necessarily mean that they will be terribly loving to you. Sometimes, in fact often when we ask to learn more about a Divine quality we are given the exact opposite circumstances to experience to help us develop that quality. So, in the Love example you might plan to be in some kind of relationship (short or long) with the soul who agreed to help you that was designed to help you learn love in extremely trying circumstances. Rather like needing to learn patience and then having things happen that really push your 'patience' buttons .
The same sort of contract can happen with an animal. Everyone who has every loved an animal knows they are special to you. We get very attached to our animals simply because they are so unconditionally loving to us. Often when they leave us permanently, for whatever reason, the contract has finished. It isn't necessarily that they don't love us anymore. It's just that it was time.
This information can come out in the Akashic Record readings. It also can come out when you are working with a good animal communicator (and NO that won't be me, although I'm learning to do this for my own sake and that of my animals ).