I think it is definitely a combination of parenting and personality. I had classic middle child syndrome growing up.
My older sister was the quiet, studious, maternal, kind hearted one.
I was the creative, dramatic, rebellious, attention seeking one (because nobody understood me of course)
My younger brother was the super smart, nerdy computer geek.
We were parented exactly the same way albeit being handled slightly differently depending on the situation. But our fundamental upbringing and rules were the same. My musical ability was probably indulged more than my sisters because I eat, lived and breathed singing and performing and my sister was and still is the most amazing artist.
My brother was the one that gave the least trouble though
As he used to say "I had nothing left to rebel against because you and DSis had done it all already!"
I would love 3 children and funnily enough can already see DS2 as the classic middle child because he is so much like me.
But no "syndrome" - I'm probably the closest emotionally to my parents, especially since we lost my younger brother a few years back, despite living overseas. My sister lives close by to them but we're different emotionally I think. I'm very heart on my sleeve, and she is very private so when DBro was sick, we worked well together to help him and our parents. She was the pragmatic one, I was the counsellor.
I'm still the peacemaker now and yes, my over dramatic side has most definitely calmed down!