I feel like I'm heading that way. Jupiter in the 12th trining Uranus/Neptune 4th, Sun, Mars and NN also in 4th, Scorpio Venus conjunct Pluto and Moon opposite Aqua Saturn. Very reticent, dreamy, detatched and unable to deal with too much human contact. I was somewhat of an urban hermit the year after highschool. Went to college, came home, rinse repeat. Socially active now, but who knows for how long.
I've become a hermit late in life - since Pluto in Cap in fact. I never was in the past, or not for long and only when very depressed and full of fear - Saturn Rx opp my Cap Sun/Venus
I think being a hermit can have several causes - At the extremes, some people prefer their own company and don't need other people or even like them much; others have learned to fear them and are protecting themselves from further pain.
I've been a hermit since 2006 but starting to come out of it now. I think Saturn going into my 12th, and Neptune through the 4th and opposing packed Leo 10th had something to do with it. Friends, work, everything just ... disappeared.
Coming out of it now, a little at a time. BP, I agree that sometimes a person just needs to pull into their shell to protect themselves. I've done it before in different periods of my life - they didn't last as long as this time, though. Also I think the double Scorpy-ness needs this every once in a while. Growth through purging, in private.
I prefer to think of it as my 'sabbatical' now. I learned a lot!
neptune on the IC, pisces DC and a packed 12th house.
love my own company and require it to feel balanced.
gemini mars on the MC can socialize easily, but i am most certainly a loner dottie.
Cancer IC and a Virgo stellium.
My progressed chart has a packed 4th house with a four planet Virgo stellium (Mercury conjunct Mars & Sun conjunct Pluto!) and is ruled by Mercury (progressed ASC in Gemini).
Like you, FT, I became very reclusive when Pluto crossed my DSC - I was 13, I think - and that continued as Pluto went on to square Sun/Mars (my chart ruler).
To lots of people, wanting to be by yourself in your teens and 20s is just plain old weird. But I needed to be alone during Pluto's long journey through Scorpio, which covers almost all of my 7th house.
I became ever so slightly more social when Pluto went into Sag, but to this day, if I don't have to be around lots of people, I'm happiest.
People don't think of Aries and Leo as loner signs, and they ARE very social, but they will go their own way/forge their own path, especially Aries. I never "followed the crowd" and never will. Guess that's why I'm not too "popular" with the drinking crowd in my neighborhood. 
Can't reply to everyone, but these are some nice answers.