"Depletion. Famine. Entropy. Desolation. Complacency in the face of those evils." He had a family, before this one. A mate. Children of his own flesh and blood. All gone now. "My homeworld teetered on the brink for so long, further and further out of balance each year. I saw the end coming, long in advance of its arrival, and offered a solution. But our leaders did nothing. Banished me for acknowledging an inconvenient truth. There had been hundreds of billions of us there, when I entered the world. Now, because they would not listen, I am the last of my kind."
It used to be so difficult, recounting this story. Acknowledging his failure to change his homeworld's destiny. Nearly five millennia later, though, Thanos has grown hard in the face of his old pain.
"I've explored the galaxy in depth, and sent scouts to a dozen other galaxies besides. The situation on my homeworld, as it turns out, is far from unique. The entire universe has grown unbalanced and unless someone intervenes it will eventually reach a tipping point."
Thanos grabs the nearest exercise machine and gives it a gentle push, tearing it halfway loose from its moorings and leaving it swaying precariously under the strain of its own weight.
"And so I have scoured countless planets: for information, technology, allies, resources, artifacts... whatever I need to restore the balance. Whatever I need to save the universe."