10) You'll get into Heaven (or, if not, at least into one of those trendy clubs you like, the next time a bouncer debates about letting you and your not-really-cool-enough-seriously-you've-got-to-be-kidding outfit in the door. Really. You will. Because Karma works that way.)
9) You will ensure that the blogger will not cry. Today.
8) You'll avoid feeling guilty about making bloggers cry.
7) The blogger may be so grateful, he/she will put you in their next novel/short story as an awesome, heroic character. Conversely, if you read a post and don't leave a comment, he/she will find out (oh yes, yes they will) and may also put you in their next novel/short story ... where you will die a coward, hiding in a port-o-johnny which then explodes, spewing forth bits of you mixed with ... well, you know.
6) The blogger may, after they become well-known, well-read, well-loved, and disgustingly rich, invite you to stay in their beautiful condo! In a space station overlooking Mars. Yes, for some of us it may take that long to hit the big time. But we'll get there!
5) You'll get practice at writing, which can always come in handy. Such as someday when you need to write out invitations for your wedding to a multi-bazillionaire member of British Royalty, who asked you out only because he/she saw the fascinating, kind, insightful comment you left on someone's blog.
4) Every time you leave a comment, a fairy gets its wings! No, actually that's bullpucky. A gargoyle gets its wings. And fangs. And claws. (And they like comments. Needless to say, you wouldn't want to make them angry.)
3) Because (hopefully, anyway) the blogger will return the favor. (They should, because it's widely known that gargoyles find blogger guts especially tasty.)
2) Because if you don't, the gremlins in your computer will be tempted to pee on your video card, and streaming movies will become like the olden days when people "watched" radio.
AND THE NUMBER ONE REASON IS ...
1) The blogger may finally be able to get his/her mother off their back by pointing to the many comments on their blog and saying, "See ― I AM SO a writer!"