Validating the Author
Every time you read another writer's post and leave a comment, you are validating the writer. You are saying - I have read and enjoyed your work, and found it to be entertaining, informative, or helpful. This encourages us to continue writing and to put forth our best efforts. So when you read something you really enjoyed, that made you laugh or even cry, please let the writer know.
Not only are we validated by comments, but the feedback will help us with future posts. As we build a relationship with our readers, we gain insight into what they relate to, or connect with the most. With this knowledge, we can write more effectively to our audience, and our posts will have greater meaning than if we just write 'hit or miss' and hope someone finds it interesting.
As you receive and respond to comments, conversations spring up, and friendships are formed. Although we may never actually meet our "blogging friends" face to face, the friendships are real, encouraging and supportive.
Promote your Blog
Here's where we get to the "me" part. It's the old give to get concept. Commenting on others' blogs will likely get you more comments on your own blog. If you leave a sincere comment on someone's post, along with your "return address", they will probably come visit your blog, and hopefully leave you a comment in return. The more active you are with reading and commenting, the greater the opportunity for others to discover you.
Whether given or received, comments provide helpful feedback, insight into your audience, new friends and, ultimately, more comments. So next time you read something you like, give the writer a heads up.