Love.

How is it that grandparents allow almost anything from their grandchildren?  Do they care more about their happiness than their own?  I would say so.  Here are a few examples of this. 
 

Pappaw allowing Ansley to gauge his eyes out

His hair was next (sorry it's so blurry)

BB hunched over walking Ansley around until she had enough

I'm sure he will later need back surgery :)

But for a smile like this...I'm sure any of us would do it!