I was going to say the same as JunkDNA. Sometimes we just can't know exactly why something is as it is. Sometimes we just don't have the memories or the information to know. We are so very complicated and it could originate in a number of experiences.
What is important is learning to accept these feelings and rather than trying to change them, to manage it in a healthy way. Working to find ways to soothe these very young feelings with self compassion and patience.