Before I had grandchildren, I never thought that I could ever love something/someone as much as I love my own children. However, when my GCs were born, the instant love and emotions I felt for them took me by surprise.
It is, however, a different kind of love to the love I feel for my own children. I think, as NotSpaghetti says, that it is the overwhelming joy that we feel for our own sons & daughters when they become parents themselves, and have the wonderful gift of their own child to cherish.
I couldn't love my grandchildren any more than I do, but it is different to the love I feel for my own son & daughter.
It is also completely understandable that friends who are not grandparents themselves do not 'get it' when you gush about your GD. I too learned very quickly not to bore people with talk about my GCs. Most other people will not be interested, although some will politely ask for updates ... but try not to go into too much detail!
A friend of mine recently became a 1st time GM, and prior to this she had absolutely no interest in either becoming a GM, or in hearing about anyone else's GCs. She too has now confided in me that the love and emotions she feels for her new GD has knocked her sideways, and she now understands what I tried to tell her.