It's first talked about in-universe on Stargate SG-1 S8E2:
O'NEILL (sitting up with Daniel's help): Getting my head sucked by one of those damned Ancient head-suckers. And something about twins. (He looks at Teal'c.) Teal'c -- what's with the hair?
O'NEILL: Thor. You got aspirin?
THOR: You should feel better momentarily and your memory will slowly return. (Jack nods, then looks at Teal'c again.)
O'NEILL: Teal'c -- what's with the hair? (Teal'c opens his mouth but Thor interrupts.)
Of course, Teal'c never gets a chance to answer O'Neill.
However, in the Atlantis episode "Midway", we see this exchange:
SHORTLY AFTERWARDS. Sam and Teal'c are walking through the city.
CARTER: So, Teal'c, the hair?
TEAL'C: You do not like it?
CARTER: Oh, I love it! It's very ... Earth!
(They smile at each other, then he looks around the area.)
Which perhaps gives us the best insight.
Teal'c has long been living on Earth, but has not successfully become part of Earth culture.
We see him try to continue to assimilate in SG-1 S8E7, where Teal'c gets permission to live off base in his own apartment. I see these two things as a very clear sign that Teal'c is ready to let go of his Goa'uld-dominated past and make a home with the Tau'ri.
I also thing the lack of a direct in-universe answer in these first two quotes is supposed to be an answer. His change was noticeable to everyone, but overall not something that necessarily needed an explanation.
Out-of-universe, we have a very clear reason given:
"Finally, after years of behind the scenes begging and badgering, actor Chris Judge got his wish -- and Teal'c got hair. Brad had long-resisted Chris's requests but finally broke down since it seemed this was going to be it, the show's final season. I didn't think it was such a big deal, especially considering Teal'c had undergone other notable changes over the course of the series run. Remember that gold tinge his skin possessed, making him look like he was a professional dancer working an all-night rave? Given the fact that certain scenes from this show's first four episodes were shot out of sequence, Teal'c's hair is somewhat inconsistent -- but I'm sure Chris will tell you it was a small price to pay for not having to shave his head every morning.
(Writer / producer Joseph Mallozzi, in a post at his blog), source GateWorld
Other interviews with Christopher Judge show that shaving his head every day for the show was actually painful, so he was quite ready to give it up.