As Rowan pointed out, the other kids went to preschool three days a week when they were her age. I don't remember feeling bad about it then.
I think it's that there is no option. That even if she's feeling bad and sad about friends/teachers/whatever, I don't have the option of just hanging out with her at home. I'm committed to work.
It's interesting because once she's at school I won't feel quite the same - she has to go to school, so there's not the same feeling of preschool being an added extra.
I do like working, but I feel sooo torn. My kids have made me a softie but actually I like it. It keeps me constantly assessing what I'm doing in light of what their needs are - that's a good thing. I'd be more worried if I didn't give a stuff.
I'm glad I didn't have to go to work when they were babies - I would have been very miserable.