Jimmy seems to think very highly of himself before he encounters Annabel Adams in Elmore and falls in love at first sight. He is young, good-looking, smart, worldly wise, and well connected with important people in the underworld and in the respectable world at large. O. Henry refers to Jimmy's dapper clothing several times. For example, when Annabel first sees him:
She lowered her eyes and colored slightly. Young men of Jimmy's style and looks were scarce in Elmore.
And when Jimmy registers as Ralph Spencer at the Planter's Hotel:
The clerk was impressed by the clothes and manner of Jimmy. He, himself, was something of a pattern of fashion to the thinly gilded youth of Elmore, but he now perceived his shortcomings. While trying to figure out Jimmy's manner of tying his four-in-hand he cordially gave information.
When Ben Price started to investigate the series of burglaries Jimmy has pulled off since being released from prison, he says to himself:
“That's Dandy Jim Valentine's autograph."
Jimmy would have been described as "dapper" in the old days. He is always careful about his appearance and his manners. He is always cheerful and pleasant. Everybody likes him. Even the prison warden likes him. He tells Jimmy:
“Now, Valentine,” said the warden, “you'll go out in the morning. Brace up, and make a man of yourself. You're not a bad fellow at heart. Stop cracking safes, and live straight.”
Jimmy's brains, vitality and personality explain why he is successful as a criminal and also successful when he decides to go straight. He knows he is at the top of his profession as a safe-cracker, but he is aware that being at the top could lead to his downfall because it makes him all the more conspicuous. That is why he moves to Elmore and thinks of setting up a shoe business in that town as a "front" from which he can travel to surrounding states to pursue his real profession of breaking into safes.
But he didn't expect to fall in love with Annabel Adams. She makes him see himself in a different light. He is a crook who has to live in constant fear of the law. He can't remain a criminal and expect to win a girl like Annabel. He chooses her, and the honest life she represents, over the pretentious but shallow and transient life he was leading before he met her. He finds that the same things that made him a successful criminal can also make him a successful businessman, with the added amenities of having a beautiful wife, home, family, friends, stability, respect, and freedom from chronic fear of being apprehended by the law. Annabel makes him realize that, though he may have been envied by others, he was leading a empty and precarious existence.