It's unclear to me whether Leno wanted to "retire" or if NBC asked him to. There's some indication that NBC asked him to, in order to hang onto Conan by promising him The Tonight Show in five years' time, and they now want Leno back because Leno's primetime show is performing poorly and Conan is performing poorly in comparison to when Leno was on.

I fear that this may be the end of a fifty-five year dynasty. I can't imagine any viewer being pleased with any outcome at this point.