You are right, there are many reasons to leave a comment. Frankly, I think most of my comments can be classified into 3 reasons.
1) I know the author and have enjoyed their work in the past and desire to encourage them or perhaps just to thank them for their work.
2) I don't know the author but their subject matter is of interest to me and again I hope to encourage them to continue and/or to say thank you.
3) Sometimes I hub surf as an exploration into our community and I leave comments to encourage new hubbers or hubbers that intuitively, I feel could use some encouragement!
In any of the cases above it is my heart to let the writer and their readers know how grateful I am for the privilege of reading their work, for their time and to let them know anything can happen if they keep trying!
Thank you CloudExplorer, I am grateful to you as an involved hubber and someone with purpose!!
Peace be with you!