Good question. If your main goal is to make money as fast as you can, then upvoting your comments is a way to do it. The problem is, it sends a message to the bigger fish that you're in it for the money. The people who have high reputations and a lot of SP have spent a lot of time building up and contributing to this platform. they want to support people who are here for the right reasons. Upvoting your comments just puts a bad vibe out there. This is just my opinion, and you can do whatever you want. I try to reserve most of my voting power for the people who read and comment on my posts. I find I make better connections with people that way.