If you want to build up and maintain a calm and steady spirit, it is a common tenet of philosophy that you have to avoid attachment to the things of the world.  This is why monks retreat from society and go to the mountains for self-cultivation.  Of course when most people think of detachment from the things of the world, they think of material things, but it is also necessary to avoid excessive attachment to living things.  They are all subject to the cycles of growth and decay. In the case of a pet, the cycle of growth and decay can be as fast as 10 or 20 years.

In addition, people are subject to their own fate or destiny.  People can change their own destiny with effort, but most don’t make the effort.  You can help someone change their destiny, but you can’t do it for them.  If you become attached to changing someone’s destiny, you are likely to suffer disillusionment.

Excessive attachment to another person or a pet can make your spirit vulnerable to their fate.  Thus we must be moderate in our attachments.