Why do we love our pets so much lol?

I have had many dogs in my life. and everytime one of our dogs dies we always say we will never get another one but we ALWAYS do. my first dog we had was shetland sheep dog named tobi. He was with me from the day i was born till i was about 13. He got really sick at the end with the tumors and such and it got to the point where he couldnt even eat. my mom said to put him to sleep to end his suffering and me being young at the time was mad cause i loved tobi so much. Well my mom said he was suffering too much and we had to let him go so i went to school that day with a bad depression. He was due to be put down at 11am, which is when i happened to be the time i was in gym. as each minute ticked closer to eleven i got depressed i came home that day hoping my mom would change her mind but when i got home she said i had to do it and i cried. and my parents said no more dogs. well about a year latter my dad came home with another dog an italian greyhound named Zip. Well of course i got attached. My mom wasnt too keen on the dog at first but then she warmed up to him and fell in love. Then in 2004 my parents got seperated and my dad took zip away. i felt sad again that was another time of heartbreak and it was hard. Again my mom said no more dogs. About 5 months later me and my brother talked my mom into getting another dog. So we did her only condition was it had to be a border collie cause she always wanted a collie and it had to be female cause we never had a female dog and she wanted one. So we got a dog named Mady so the first day she came home and cuddled right up to me and instantly i got attached again. Now Mady is almost 6. she stole all of our hearts especially moms. now we are really attached to Mady i dont see her as just a dog she is one of our family members, i call her my older sister. Why must we all get so attached to our pets. not saying its a bad thing but its gonna end up in heartache again when Mady dies. And i dont want Mady to die cause like my dog tobi and when zip got taken away it will be the worst day of my life.