The reward pool is the same amount every day regardless of how voting takes place. So by upvoting his own effortless comments, he is actively REMOVING reward from someone else who worked hard on their post. By rewarding himself he is taking away from others.

This action if allowed to continue promotes such behavior in others. "If this guy can make a bunch of money upvoting his "Thx" comment, then why shouldn't I?" Once the rewards are being monopolized by people who are making money with little to no effort, then the genuine content creators will leave because they are not getting rewarded.

When genuine content creators leave, and investors come looking at Steem and see a pile of crap posts, meme's, and people getting big upvotes on effortless comments they will decide not to invest. Without people wanting to buy in to Steem the value of the Steem token goes down. This hurts across the board, whether you have 5 STEEM or 500,000 STEEM, if the value is Zero, your value is zero.

So long answer short, promoting GOOD content promotes a higher value of Steem, promoting crap content contributes to the downward pricing influence, and, in a complete doomsday scenario, kills the platform when Steem value drops to an effective Zero across the board.

Good Content - Good For Growth
Crap Content - Good for short term, get in and get out profits for people who don't care about the longevity of the platform.