The retweet is a proxy for immortality, which is why we love it so much
"What we do in life echoes in eternity."
This highfalutin slice of swashbuckling nonsense comes from Russell Crowe's portrayal of a Roman general in Gladiator and is precisely the sort of thing you'd expect to hear yelled at a bunch of dudes being led to their death for the sake of expanding some uncaring regent's territories and wealth. You see, few things are as demotivating as the prospect of imminent death, so if you can stir up some bravado in the troops by promising them afterlife resonance, you do it and you close your eyes to the absurdity of what you're saying. The promise general Maximus was making to his men was simple: you may die physically today, but history will remember you forever and thereby grant you a type of immortality.
So long as your story is being retold by someone somewhere, the significance of your existence cannot be entirely extinguished. Never mind the fact that you're already decomposing.
The desire for, to put it crudely, more life is deeply ingrained in all of us, to the point where those who profess indifference about their future survival are considered psychologically imbalanced. We're all kinda, sorta aware of the fact that practical immortality is impossible (to quote Chuck Palahniuk, on a long enough timeline, the survival rate for everyone drops to 0), but we maintain a subtle veneer of denial in aiming to survive for as long as possible. It's just an instinctive drive to ignore the facts and keep striving.
The natural predilection for things that promote the extension of life manifests itself in direct ways — eating and sleeping to regenerate our bodies, moving away from crime-riddled or polluted areas, avoiding physical conflict — as well as increasingly indirect ones. For example, you might have children, at least in part, so you can continue the lineage of your genomic data, or you might endeavour to perform some universally laudable feat so that your name could reside in the annals alongside those of Gustave Eiffel, Louis Pasteur, or, hell, Mikhail Kalashnikov.
This is where Twitter's seemingly innocuous RT comes in. Like Maximus' echo in eternity, a retweet is a reverberation of whatever you might have said, prolonging (oh so briefly!) the exposure, relevance and significance of your words. It touches on the basest of human instincts and delivers a deliciously stimulating high when it ratchets up into a chorus of multiple RTs. It's like an orchestra of the world celebrating both your existence and the genius of your tweeting. You're simultaneously flattered and encouraged to believe that what you're doing on this planet has some meaning or value or purpose. Which is sweet.