Continuity mistake: Dexter's hybrid SUV changes often. Sometimes it has satnav, sometimes it doesn't.
Continuity mistake: Dexter browses the book shelf at Travis' house. The camera pans across, revealing a book titled "Apocalypse Bound" situated at the right on the shelf. When Dexter takes the book off the shelf, it is then shown as the fourth book from the left. When he puts it back, the empty gap where it used to be now appears to the far right on the shelf. (00:25:50)
Just Let Go - S6-E6
Continuity mistake: When Travis meets his sister at the school she works at, a chalk board with nothing on it can be seen behind Travis. A couple of shots later and there is a piece of paper attached to the chalk board. (00:30:35)
Continuity mistake: In Season 6, Episode 10, Dexter and Harry are in the motel room where Travis was staying (timestamp 1). Dexter decides to post on Gellar's blog (impersonating Gellar), and the screen on the computer shows the post date as Sunday, November 24, preceded by a Sunday, November 20 post. Aren't Sundays supposed to be a week apart? Then in episode 12 (timestamp 2), Travis is in Dexter's apartment and sees the poster for the Noah's Ark school play. The date posted for the play is Saturday, October 8, ie. in the past compared to the earlier episode. Plus If November 24 were on a Sunday, then October 8 should have been on a Tuesday. (00:09:20 - 00:30:20)
Answer: Two reasons. First, as a cop he knows that there are lots of murderers walking around free due to flaws in the system so he uses Dexter's "urges" to right the wrongs in the system by taking out people who have no right to walk around free. Secondly, by taking him to a psychologist he runs the risk of having him committed and he doesn't want that for obvious reasons, so he figures it is best to channel his urges to what he considers a greater good.
The_Iceman
In addition to the earlier answer, in the last season (8th) of the show, we are introduced to Dr. Vogel, whom Harry had been discussing the situation with Dexter as well, and it is shown on-camera that Harry describes what Dexter has done (killing)... So, technically he did consult a psychologist/psychiatrist, just not having Dexter and her actually meet. But Dr. Vogel was aware of whom it was and how to intercede.