like many others, i believe grizabella left the tribe to see the outside 
world only to find it cold and cruel to her so she returns to the tribe to 
be accepted again.

BUT!!!

grizabella could'nt have left the tribe without a very good reason bcoz the 
tribe seems to be a fine one. my theory is that someone did something to her 
and she was hurt by the experience so she tries to find refuge some other 
place, which is the outside world. she encounters many cats, among those 
macavity. grizabella sees how macavity treats his prisoners so she thinks 
"where do i go now?" and she escapes from him and wanders off. during this 
time, demeter (who is mac's captive) sees her and feel sorry. after a long 
time, grizabella decides to forgive and forget, and returns to the junkyard. 
but the cats get the wrong idea about her so they reject her. demeter 
remembers who griz is and tells the others about her, hoping to change their 
opinions on griz. but only jemima understands and sympathizes with griz.
i think there's a strong connectoin between grizabella, demeter and jemima. 
see, dem tellas about griz and jem helps griz.
plus, during the journey to the heaviside layer, demeter gives grizabella a 
look which seems to say "I'm glad you made it. I hope you finally become 
happy now." something must have happened between them three!!!

but these are just my opinions on Grizabella (as well as dem & jem). i 
mostly belive what the others think about her, but everyone's entitled to 
thier own opinions. i just expanded a lot the main idea, heehee...

jemima etcetera