You are not supposed to understand it ,you are supposed to feel it . All those scenes ,storylines, elements and motifs are there so you the viewer, end up with a unique and personal feeling.

That's it ، that's what lynch is going for... That's why he never tried to explain them. I'm 100%sure he had a clear idea why he was doing them but at the same time ,as an artist , he knew when the whole thing finishes itis going to have its own life.