Concept of the "Eternal sunshine of the spotless mind"

This movie is one of the deepest psychological romantic stories ever. It goes to the very root of what we call romantic love which is actually completely internal/part of our dreams. We build a whole association network with the person loved.

After going through a variety of emotionally charged experiences with that person, the experience itself starts getting linked to the internal impressions of that person.

• You associate all intense pleasure/pain sensations to the lover and he/she starts becoming indispensable to your life experience itself. 
• The lover acts as a catalyst in bringing out cherished or emotionally charged states in you and they get intricately woven into the fabric of life experience itself. 
• Beyond external experiences, we also start attaching this person to our fantasies and this makes this network grow into even your most primal drives in life.

That's why a common feeling after a breakup is that, I cannot/will never feel alive again. It requires a process of disconnecting/detaching the person from your life experiences or replacing them with someone else. For it to be replaced, the new experience should be more intense. More the intensity, more is the strength of the memory and association.

In the last part of the movie, near the seashore, he suddenly comes into the present moment and realizes all that he missed out. Becoming a pure observer, savoring each feeling's intensity and letting go, detaches you from the situation/context/person and gives full freedom to the experience. The experience is no longer locked into the pattern that only a certain person/context/situation can invoke it.

Becoming this observer frees up your mind into the truth and makes it spotless and you become what you always were and will be. Hence "Eternal sunshine of the spotless mind".