Why someone becomes pedophile?
At the current moment, the answer is unknown. There are different theories. There may be genetical or hormonal influences or early childhood experiences. It seems reasonable that homosexual and heterosexual pedophilia has to be considered separately.
This question has to be clearly separated from the question of reasons to make sex with children for pseudo-pedophiles - people who have sex with children but who are not pedophile. Unfortunately, many researchers have ignored this difference. As a result, the scientific value of a lot of research is questionable. The very popular "transmission theory" (sexual abuse in the childhood as the reason) is an example (Reeves 1992).
Nonetheless, there are some empirical results which allow some suggestions.
Freund (1993) suggests that "erotic age preference" is determined at a fairly young age, no later than puberty, and very possibly earlier than that.
Blaske et al. have studied ( 1989 ) "individual, family and peer characteristics of adolescent sex offenders and assaultive offenders". Their primary finding was that while assaultive offenders hang out with a peer group which is also delinquent, the family relations and behavioral functioning sexual offenders, on the other hand, were much like the nondelinquent adolescents, the main difference being that they are more isolated.
One interpretation of this data is that assault is a behavior which is strongly influenced by a peer group, while sexual offences are more driven by internal processes.
The autobiographical reports of pedophiles I have also indicate that the orientation is determined at a young age: "My own sexual 'persuasion' is something that I have felt and grown up with from as far back as I can remember - perhaps as young as 5 or 6." (James)
Considering the distribution of pedophile feelings, we find that pedophilia is not a yes/no state, but that there are pedophile feelings of lower or higher degree. This suggests that there is no single "pedophile gen", but more complex reasons.
We can consider man-boy love as an extremal case of a normal bondage between adult man and boys. From this point of view it is interesting to study this association between adult males and prepubescent boys. The results of Mackey (1990) suggest that this is a common human trait, independent of culture, thus probably genetically transmitted. Selective pressures connected with the onset of hunting and scavenging as male-only prerogatives for the early Homo are suggested as the reason: "... older males recruit peripubescent males into all-male (e.g., hunting and scavening) groups." "It is reasonable that the sharing of a most valuable commodity - food - is much easier for the provisioner if the provisioner 'likes' the recipient, and conversely, a recipient tends to like someone who feed him or her."