I agree with what Blondie Bear said, but also want to add, it was a case of 'fitting in' as well. At high school, Harmony had her group that she fit into... she had her 'place' in the 'world' (the world being sunnydale high, cuz I doubt her imagination drifted far beyond school).
And like any popular person with a following at school, when it ends, you struggle to find where you fit in- you go from being someone to no-one... for Harmony, she had the added problem of being a vampire on top.. she tried to be a big bad 'goff' style vamp, and it didn't work, she tried to be the sex-kitten vamp (with Spike) and it didn't work, and so off she went to LA (just like Cordy, who was trying to find where she fitted in the world, for similar reasons)... and I think that she found her place, being a kinda un-evil vamp... I mean, she always references wanting to drink human blood, but she's capable of not doing it, in accordance to 'fitting in' with her 'place' in the 'world' (W&H), which is what she wants more than anything.... I mean look at the episode in season 5 (the name escapes me, sorry) where none of the other girls at work talk to her... thats a perfect example of her trying to fit in, make friends, be popular and basically replicate her high school status- thats her mission in life, as opposed to Angelus/Spike's of death, havoc and mayhem.