My 'beginnings' were very similar, Jen ... my sister (half-sister) was born when I was 5 and of course all the attention went to her. I'd just started school as well which, for a 5yo, is an alien environment so you need attention when you get home but i didn't really get any. So, i found myself wishing i was her/that i was a baby.
(I have very vivid memories of standing on a step, looking into the cupboard which had all the baby things ... plastic pants, baby powder, cloth diapers and pins and the intoxicating smell & feel of the plastic pants)
As she grew up she still was "the favorite", getting all the positive attention. Being jealous/fed up of that I tried several times trying to get her into trouble which always back-fired - i found my parents left a tape recorder in her room and recorded me teaching her naughty words - so all I got was either negative attention and/or punished.
Years later I found that when I was a baby both my parents worked and i was left with relatives so i never bonded with pretty much anyone in my family and was, before i found this out, somewhat resentful.
In my mid/late teens i spent a couple of years reading sexual psychology books to try to understand myself, especially where the 'wanting' humiliation/embarassment etc - and an emerging want for bondage etc - had come from but never found it. Recently, a friend said that it was likely that all i wanted/want now is some attention.
It is quite possible that people who do not get the "right amount"/positive attention in their lives may have part of their persona become stuck at a younger age and it could become a problem (perhaps) as years pass.
Regarding the humiliation etc, personally this has gotten stronger & stronger in me over the years - maybe from not finding someone to be a 'parent'/dominant etc or maybe that i haven't accepted myself as an ab/"something". (I did have a Dominant for a while a number of years ago and said one time after we played that i could see a day when i didn't need to "do this stuff any more", i.e. that it might be a way for my 'inner child' to heal. Unfortunately things didn't work out for us.
I will say though that the ... need/desire for the (negative) attention i.e. humiliation/degradation etc has the possibility to become destructive as one could become masochistic or very depressed - or both.
Sorry for the long blurb. Jens' post brought out a need, i guess, in me to share how things started for me.