ET usually is a form of maternal and/or paternal transference. In psychoanalytic theory you have pre oedipal and oedipal phases; pre oedipal being the mother and baby period, where you have your first experiences of intimacy... breast/bottle feeding, touching etc. These are nurturing acts but are also experienced in the same way arousal & sexual feelings are. As a baby arousal, pleasure & joy are much the same.

For girls, the oedipal/Electra complex is where you would begin to move away from your mother and look towards your father for affection, around 3/4 years roughly.

If something goes wrong during one or both of these periods, it may affect relationships later on down the line. It may also play out via ET in therapy. For instance, you may not have had the proper care as a baby and keep seeking that lost loving care or had a father who didn't return your affections... cue loving your therapist who is unavailable and unable to return your love. There's lots of reading out there. A really good book (though can be tricky to obtain) is 'In Session' by Deborah Lott. She explains the reasons behind ET and talks to lots of women about their experiences.