Why domestication seems to decrease animal's life span?

This is just another example: Research shows that most domestic dog average life span is about fifteenth years. This is of course,different from their close relative,the wolves,which may life for more than 25th years in the same condition(in captivity).
I’m not talking about size here. As we know that smaller dogs tend to live longer than bigger dogs due to the fact that they don’t need to support bigger activities in their body just like bigger dogs. But even so,if we compare a dog and a wolf from the same size and same living condition we’ll see that wolf tend to live longer.

So why domestication seems to decrease animals life span despite their less hazardous and well taken care lifestyle by human? Could it be that because of they lack of genetic diversity that shorten their life span?,evolutionary impact?,or there could be another explaination? Hmm…. I’m curious here.