The historic explanation of the use of “se” instead of “le” or “les” is very complex.
First, the Latin sequence “illi illu” suffered a series of phonetic changes that produced “ge lo” in medieval Castilian (Pharies 117-118).
Second, “ge lo” was confused with “se lo” in 16th century Castilian.
Another, more simple, explanation (but completely false) is that the “se lo” came to pass solely to avoid the cacophony of the “l” sound in syllables too close together. This theory follows that this construction simply sounds bad and is a little unnatural in spoken Spanish (Pharies 117-118). However, given examples like: <<…voy a decirlelo lo lelo que es…>>, it is actually easier to say this than its modern counterpart: <<…voy a decirselo lo selo que es…>>
Pharies, David. Breve historia de la lengua espanola. Chicago , London : The University of Chicago
