One can write:*glielo* porto, *gliene* riferisco, *gliela* scaldo, ...but not:*lelo* porto, *mene* incolse, *tela* scaldo, ...Why *gli* is different in this respect? Where does this difference originate?
The main reason why italian as combined pronouns forming a single word: GLIeLO, GLIeLI etc is for phonetic reasons. Italian is phonetic 100% and needs to flow as a wave otherwise phonetic pause could be interpreted as double consonant.
i.e. GLI LO do' domani, phonetically saying GLI LO does not make a round sound and if we pause in between it could be interpreted as GLI-LLO, so the solution is to add an "e" in the middle making one word and the sound of GLIELO is a soft wave.