Twenty years from now you will be more disappointed by the things you didn't do than by the ones you didhas naturally gained quite some popularity.
As I look back at the last ten years of my life, I can think of not just things I should have done but at the possibly different persons I could have been. And along with that would come a very different set of experiences that I could have had. This notorious what if situation is quite an emotional killer. When I initially had the urge to write about this topic, I searched online for that exact quite by Twain and discovered a more fuller version which goes as follows
Life is short, break the rules, forgive quickly, kiss slowly, love truly, laugh uncontrollably, and never regret anything that made you smile. Twenty years from now you will be more disappointed by the things you didn't do than by the ones you did. So throw off the bowlines. Sail away from the safe harbor. Catch the trade winds in your sails. Explore. Dream. Discover. That gave it a whole different meaning.
I've always maintained the view that life is actually long and not short. It is because life is long that we get to reflect back and ponder over how different it could have been. The trick is that though life may be long, I believe there is such a thing as the best phase to do certain things, and that is the root cause of regrettable inactions. It is not that I cannot now do what I think I should have done 10 years ago. It is that even though I may do it now, I believe it won't be a tenth as good had I done it back then.
The severely understated part of the whole idea however is never regret anything that made you smile. As the fabled Albus Dumbledore says
The choices we make create the lives we lead. By not choosing to take the myriad different paths and not doing the limitless number of things we could have done, we would have actually done something and taken a certain path. And if that path made us smile in those moments, then we should probably let them be; for what our emotions fail to do is to illustrate what we now take for granted would not have been available to us in that fantasied alternate universe that we so lament not being in.