Now, we have most of the movie centered around Wall-E following Eva around, in an attempt to be of some help to Eva as she fulfills her directive: That of bringing a plant to the commander of the Axiom. For Wall-E this isn't so bad; he is so smitten with Eva that only her happiness and Directive matter to him.
He has, in essence, grown out of his own directive, that of compacting trash, during the long lonely decades down of Earth.
When Wall-E ends up broken and dying in the Axiom garbage disposal facility, with Eva there alongside him, he still tries to give Eva the plant in order for her to fulfill her 'Word of God', her Directive. While Eva, having been forced to review the recordings of what happened on Earth (and while she had been shut down) and along with Wall-E getting severly hut she has come to realize that she loves Wall-E. But Wall-E is about to die and she is trying to fix her, desperately searching for parts. But since Wall-E is a 700 years old model greatly out of date, Axiom naturally has no parts for him.
So, dying and unreapairable by anything on the Axiom, he still offers Eva the plant. Eva, instead of following her Directive, pauses to think. She looks at the plant, throws it away as inconsequential. She then looks at Wall-E and declares him as her new Directive, placing Wall-E as her new reason for being and existing.
She, essentially, went against everything she was programmed for and she did this in a day after coming back from Earth. A day, with Wall-E, was enough to make her defy her Directive. Wall-E had decades, centuries down on Earth. For him the process was gradual.
Wall-E made Eva go through that in a day. Talk about love.
Later, when they are back on Earth and Wall-E has suffered some more and therefore shut down, Eva fixes her beloved. But here comes a philosophical question:How much of a robot can you replace with new parts before it stops being the same robot you began with?
Wall-E was banged up bad, inclusing some of his circuits. A slight disappointing surprise then for Eva when Wall-E upon reactivation beings to haul garbage without even a flicker of personality evident upon him and no apparent recollection of Eva.
Eva, after trying to make Wall-E remember and failing, is crying for her lost love. As far as she knows and can see, her Wall-E has died. She takes hold of Wall-E's hand (for the first time in the movie as a romantic gesture) and decides to give one last robotic kiss to remember Wall-E for and as a kiss of goodbye.
However, as she is about to leave, Wall-E will not let go off of her hand. His eyes flicker as if something is rebooting. His circuitry has been changed, most of the new him are made of spare parts and by all accounts he should be dead. But an electronic kiss, a small electrical current if you will, immediately rendered all that moot. His eyes turn upwardsto face hers and he asks in a surprised and disbelieving (and 100% Wall-E-style we've come to know) voice: "Eeee-va?!"