Cut the psychoanalysis, Freud
One of the more annoying things about parenting is that if anything about a family's situation is "different" in any way, suddenly that is the "cause" of everything about the children. And by "different", I mean "different than the observer's situation".
So, if you occasionally spank your child in certain discipline situations, and then sometimes your child whines or plays too rough, the "cause" is the spanking. If you avoid all spanking, and your child whines or plays too rough, the "cause" is the lack of physical discipline. If you have many children and the youngest is sometimes aggressive, or is timid, or takes things, or begins talking later than other kids, the "cause" is the many children. If you have a single child, and your child acts timid around other kids sometimes, or doesn't want to share, the "cause" is the single child status. If your child goes to a private school, and sometimes acts selfishly, the "cause" is the hoity-toity school atmosphere. If your child goes to public school… If your child is allowed lots of toys… If your child is allowed few toys… If you are a single mom…. If you are a single dad…. If you are religious… If you are not religious… etc. etc. etc.
It doesn't matter if the behavior is positive or negative, in each of these situations, the child is merely acting in a certain way, and you are inferring arbitrarily and based on absolutely zero evidence that the "different thing" is the cause. If you were to see a child (eg. your own child) without that "cause", and he or she acted in exactly the same way (like you probably see children do nearly every day), it'd be just "normal child behavior" which doesn't need a cause other than "he/she is a child".
Equally annoying is when parents observe someone else's child act in a particular way (good or bad), and infer that there must be some unseen parenting difference which "caused" it.
Determining cause and effect in human behavior is extremely difficult. With differences in genetics, diets, hormones, age, climates, sleep, education, families, experiences, recent events, and about 8 billion more variables, picking out one tiny piece as "the deciding factor" is, frankly, arrogant and insulting.
This is not to say any of these things are not, in fact, related. Human behavior is obviously affected by environmental factors. We all learn from our experiences, both positively and negatively, and of course we should try to discover and provide an environment which we feel will lead to the best behaviors in our own kids. I welcome rational discussion about possible influences on a child's behavior, but human behavior is extremely complex, and you don't discover everything you need to know about any child (not even your own) merely by hearing some circumstance of their life, so all this simple-minded Monday-morning parental quarterbacking and guesswork of finding a "cause" is… unhelpful. You are, of course, free to observe, make correlations, guess, extrapolate, and learn from the successes and mistakes of others, but always remember that you don't, and probably can't, know.
So please, let children be what children are: complex human beings, not simple 3-piece puzzles to be disassembled and deciphered by amateur child psychologists, and diagnosed by every temper tantrum and peer interaction in terms of a "cause".
If there are any questions, here's a handy flowchart which explains the process I find annoying: