it is explained to a degree, in "On the Way to a Smile" the 3 part short story compilation that takes place between Meteorfall and AC... it's written by Nojima so it is canon not not some lame fan-fic... Smile clearly illustrates how Cloud started feeling more down about his past failures and didn't feel he was worth having someone as loving as Tifa and Marlene around to take care of, and it got worse after he found Denzil nearly dead from hunger & exhaustion from living in the derelect ruins of Midgar and bringing him home thinking Aerith led him to the boy, it was just a downward spiral from there, then everyone gets Geostigma and he feels his failure is complete... so he does what Cloud does naturaly and runs away to die in solitude... after all if he can't save himself or the life of one little boy who could he save?..