«The Voyage of Vanishing Men» by Stanley Mullen

The Voyage Of Vanishing Men written by Stanley Mullen Earthmen had never ventured into the vast unknown beyond the galaxy. But now a survey was ordered and a ship sent out. So Braun went on — The Voyage Of Vanishing Men.
They still talk of Braun, and the Fourth Intergalactic Survey.
Other men before him had gone out into the far, dark places. Three previous expeditions had gone out and vanished completely. Then the Venture IV went out and out and out countless miles and light-years and whatever else it is—and out there in the lonely darkness something happened. Nobody knew exactly what happened, but there was a lot of guessing. Only one man came back. Braun. And there was talk….
Tending bar anywhere is better, they say, than an academic degree in psychology. Tending bar on one of the way stations to the stars you see people—most of them human—as they really are, and in all stages of emotion. You see them coming and going, and a few already gone. By little signs, you can tell a lot about them, and make a guess at what is wrong with the wrong ones.
There was Braun.
Nobody said anything, at first.
Braun watched them, a humorous half-defiant glint in his eye. But there was pain in him, in his voice as he spoke.
"What's the matter? Am I poison, or something?"
Somebody said it, then. In a stage whisper. "I had friends on the Venture IV."
"So did I," Braun answered quickly. "A lot of friends. So before somebody works up nerve to ask, I don't know."
"Don't know?" a man named Cutter pursued the point coldly. "You were there!…"
"I was there," admitted Braun. "I still say it. I don't know what happened to anybody. I've told the authorities that over and over. I've told anybody who'd listen. You don't have to believe me. I don't give a—"