I used to think that it was failures in the family that BLs cover for. But then I had a yf whose family was very loving and whose father was very involved. After that, I had a number of close relationships (that I wouldn't consider bl/yf relationships) who had good or even idyllic families. So, I don't know what it is anymore. Perhaps part of it is the very fact we are not family and they want acceptance from people outside their family.

I believe the best way to tell if the boy you care about is your yf, is to see whether he considers you his bff.