Life swings, like a pendulum, back and forth between pain and boredom — Arthur Schopenhauer

This quote distinguishes two different states:

• Pain (or suffering) is a consequence of unfulfilled desires. We are in pain because we want something we don't have. In Shinji's case, he craves human contact but fears it at the same time, meaning he can never accomplish his desire (see the Hedgehog's Dilemna).
  
• Boredom is much harder to define. At its core it consists in a total lack of desires. This includes both Eros pulsions — those of life and creation, seen as sexual in the Freudian interpretation — and Thanatos pulsions — those of death and destruction, even of one's self. This state is typically associated with a feeling of being lost and a questionment of one's identity.

During the hospital scene, Shinji fulfils part of his Eros (sexual) pulsions. However he does so in such a disgusting way that it triggers some kind of "emergency reaction", as expressed by his dialogue right after the act. This makes him transition from his characteristic state of pain to a state of boredom.

While "bored", Shinji lacks all desire. He avoids people and folds back on himself, without enough willpower to either kill himself or fight for love. He is left in this state for most of episode 25'.

This theory gives an explanation (not an excuse, an EXPLANATION) for Misato's kiss scene. She understands Shinji's lack of desire and desperately tries to bring them back. However she only knows one way to do so, her sexuality. Her goal was (at least in part) to make Shinji act; while bored, he cannot do anything, since he lacks any form of energy (associated with pulsions and desires).

This attempt is a failure however, and Shinji remains inactive while Asuka gets killed. This constitutes another traumatic event that makes Shinji swing back to a state of pain and desire, although this time dominated mostly by Thanatos pulsions of death and destruction — hence the triggering of the Third Impact.

I think this theory is coherent with the show's Freudian and Schopenhauerian themes, and it can help explain Shinji's radical changes of mood throughout the first part of EoE. Feel free to tell me if a similar theory already exists, as I'm quite new here