Why does Willy admire Dave Singleman?

WILLY: His name was Dave Singleman. And he was eighty-four years old, and he’d drummed merchandise in thirty-one states. And old Dave, he’d go up to his room, y’understand, put on his green velvet slippers—I’ll never forget—and pick up his phone and call the buyers, and without ever leaving his room, at the age of eighty-four, he made his living. And when I saw that, I realized that selling was the greatest career a man could want.
Willy is trying to persuade Howard to let him represent the firm locally so that he doesn’t have to do all that traveling throughout New England. Willy realizes that Howard thinks he is getting too old either to cover New England or to adjust to an entirely new territory in the vicinity of New York City. He brings up the memory of Dave Singleman to show that a man can be a successful salesman even in his mid-eighties. Willy is only hurting himself by dredging up memories of the distant past which make him seem like an old-timer.
In addition to inspiring him to become a traveling salesman, Dave Singleman gave Willy the illusion that he didn’t have to plan for or worry about the future. Willy has persuaded himself that he would never have to retire. He doesn’t know anything but selling. He wouldn’t know what to do with himself if he were able to retire. If he were unable to retire, he could just go on doing what he has been doing most of his life. He believes that the job should actually get easier with advancing age because he would build up so many valuable contacts that, like Dave Singleman, he could just pick up the phone and take orders without even putting on his shoes and leaving his hotel room. Those kinds of easy jobs do exist, but they never last. Competition catches up with them.
Dave Singleman was only a casual acquaintance. There is no reason to think he was telling young Willy the whole truth. Singleman may have been working because he had to scratch out a living and couldn’t retire even at the age of eighty-four. Willy doesn't tell Howard that Singleman made a good living but only that he "made his living." In other words, he was just existing. The name Singleman is pretty obviously intended to suggest that he was unmarried and probably never was married. Unlike Willy, he didn’t have a family to support and mortgage payments to make. Singleman probably enjoyed life on the road even if he was just making a bare living for himself alone. It was better than sitting in some bleak room in New York staring at the four walls--and he did get to meet a lot of people in his travels, even though they were no longer people of his generation and may have been tossing him a little business out of charity.
Willy didn’t plan ahead because he thought he could just go on selling into his eighties. He doesn't realize Dave Singleman was an anomaly. Willy is already worn out and used up at only sixty-three. He based his whole life on a single example. If he had ever thought about what he might do in retirement, he must have seen himself puttering in his garden. When he finally got around to planting some seeds as a sort of experiment in retired life, it was the middle of the night and the vegetables couldn’t grow anyway because the surrounding apartment buildings were cutting off most of the sunlight.
In addition to suggesting that Singleman was a bachelor, the name suggests that he is a single instance on which Willy had placed too much reliance. Dave Singleman is a single, unique phenomenon. Willy met him many years earlier when he himself was a young man. If Singleman had been 84 when Willy met him, say around 1920, the old man would have been born around 1840. His life expectancy at birth would have been around 42 years. Willy's life expectancy in the 1880s when he was born would have been about 53 years.
Throughout the play, Willy is characterized as being inflexible and impervious to advice. He just shuts people out and refuses to listen. This is probably his tragic flaw: he can't face reality until reality overwhelms him. Arthur Miller once said:
The essence of all drama is this: The chickens come home to roost.