We wrinkle from smiling and aging, but also from worry. Mark Twain is saying that if you do have wrinkles, they should be from your smiles and not your worries.

Hi... Many people see wrinkles in a negative way. They don't like to see themselves getting old. The writer sees them more positively - if a person has smiled a lot, this is shown in the wrinkles (laughter lines) in his face.

I totally appreciate and agree with Marie-Helene and Peachey for their excellent and perfect explanation. so i'd like to show them both big thum up ^_^ by "Wrinkles should merely indicate where smiles have been", the writer is actually using the art of writing to persuade people into seeing things in a positive way. Just as Peachey has already well explained, we wrinkle from smiling and aging, but also from worry, why not believe in that our beautiful wrinkles come from our sweet smile, or even from our big laughter ^_^

this one is easy, u know when you smile you get some wrinkles.... so on.. you can deduce this quote