Why Boxer dog's die young

Abby is turning into a good looking dog.

Thanks...she is over a year old now, but I don't have any recent pictures. I also just rescued a 11 month old full-blood St. Bernard (Bella) a few weeks ago. My friend's neighbor was going to have her put to sleep because he was moving. Makes no sense to me because she is a very sweet and LARGE puppy. She's probably right at 100lbs now.