I just spent the weekend with several sets of grandparents (not my own) who love spending time with their grandkids.  I developed a few theories about why grandparents love grandkids: 

1.  When parents have kids, they're usually too young and/or too busy to fully enjoy the experience.  A young married couple has to worry about being married, working, paying a mortgage, etc.  Raising kids isn't exactly a textbook experience (from what I've observed), so there's quite a bit of pressure and stress involved.  Grandparents have been married for 10s-100s of years, are retired, and have paid off their house(s).  Essentially, life begins when you're old.  The only thing left for grandparents to do is enjoy it. 

2.  Grandparents are like long-term babysitters.  They come over and play with the kids for a while, but then give them back at the end of the day.  Sometimes the kids will stay overnight with the grandparents, but no matter how long they stay, there's always the thought in the back of everyone's mind that the real parents will eventually come back and take the kids away.  If the kids are cranky or being bad, it's probably pretty comforting to know that a grandparent's job is nowhere near full-time. 

3.  Grandparents don't have to discipline grandkids.  That's a job for the parents.  Some grandparents feel the need to do it, but most will hand the kids over to the parents. 

4.  They're not their kids.  When it comes down to it, grandparents are free from responsibility.  "Your kid threw a football at my face."  It's not my kid, it's my grandkid.  "Ok.  In that case, I'll talk to the parents."  Grandparents don't have to change diapers, clean up puke, feed, or bathe grandchildren.  They can and often do volunteer for some of these services (ok, really just the bathing ... bathtime is lots of fun!), but can rightfully decline without shame or remorse.  "I'm done playing with and spoiling your child; you can take them back now." #psychology