A number of Hadiths in Sahih of Bukhari state that the prophet (sws) forbade fasting on Friday if one did not keep fast a day before it or did not intend to fast on the next day I just want to understand why was this rule set?
Since, the ahkbar-e ahad (individual to individual narratives) do not always cover the context and full background of the issues they discuss the scholars do not take them independently. One has to collate all the narratives on the topic and the teachings of the religion on the issue before forming an opinion. The Muslims scholars who have studied these narratives have not concluded that fasting is absolutely forbidden on Friday unless one has fasted the previous day or intended to fast the following day. Imam Abu Hanifah and Imam Malik have been reported to have held that it is not even disliked (makrooh) to fast on Fridays even if the stated conditions are not met. They, therefore, concluded that it is allowable to fast on Friday. Imam Shafi'I, however, held that it is makrooh for such a person to fast on Friday who is weak and about whom it is feared that once fasting an optional fast he will not be able to discharge more important pressing religious duties like the Prayer. Some other scholars have held that it is makrooh. Please note that they do not take it to mean total prohibition as is clear. Even these scholars who hold makrooh say that it is allowed to fast for the one who fasts habitually three days in the middle of every lunar month or the one who has vowed to fast on for example the day his son get well after an illness and that happened to be Friday.
This means that this report has not been taken independent ruling binding on all. The scholars have tried to guess the reasons behind this ruling but no satisfactory answer has been forwarded. I believe there is not enough material ascribed to the Prophet (sws) and we cannot conclude a fully understandable answer. We can therefore, not hold it forbidden to fast on Fridays unless we have found sufficient evidence to general and absolute allowance to fast whenever they like.