Why does it always have to end like this?

Larry Bird always is and always will be known as a Celtic, Magic Johnson is and always will be known as a Laker and Michael Jordan is and always will be known as – oh right, almost forgot about that second retirement.

But Jordan playing for the Wizards, Malone the Lakers and Ewing the Sonics is becoming more the rule than the exception in this day and age of professional sports.

As a D-backs fan since 1993 when Jerry Colangelo first mulled the idea of bringing professional baseball to the desert during a certain NBA Finals of that same year not to be spoken of in these parts, it was particularly strange to watch Randy Johnson win his 300th baseball game last week.

To put it simply, Johnson belongs to Arizona, and for San Francisco fans to pretend to be excited about watching his chase to 300 made me as sick at it did that he didn’t end up winning that final game in Phoenix, although at least D-backs fan had the chance to give him a nice ovation to honor the achievement Wednesday night.

The Big Unit went from being a dominant but erratic pitcher with Seattle to one of the all-time greats in the desert, when he became arguably the greatest free agent signee in history by going out and winning four Cy Youngs during the four years of his original contract.

Along with Curt Schilling, he pitched the D-backs to a World Series victory by winning three games in the 2001 Fall Classic by himself, and he reached perfection alone a few years later.

After that ill-fated stint with the Yankees, he came back for a few years to be a serviceable if not quite dominant pitcher into his mid-40s.

D-backs fans lived and died with Johnson for the better part of this decade, yet when D-backs management decided they wanted Jon Garland instead, the Unit went on to cap off his career by becoming the 24th and possibly final pitcher to achieve “300” with the Giants.

Johnson won 118 games over eight magical years in Arizona, yet he reached that final crowning achievement with only his fifth win as a Giant.

The Unit certainly isn’t alone among stars of this era in changing teams at the tail end of their illustrious careers, and it certainly takes something away from those final career accomplishments, for the fans if not the player.

Here with the Suns, Shaquille O’Neal made a mad dash up the NBA’s all-time scoring list this season, and he didn’t stop until he reached the pantheon of NBA scorers with only four greats ahead of him: Kareem, Malone, MJ and Wilt. That foursome has been known to put the ball in the bucket a time or two.

Although baseball is a game where stats take on a life of their own – particularly 3,000 hits, 500 home runs, and of course 300 wins as far as individual career achievements go – 23 other pitchers welcomed the Big Unit into that elite club.

Shaq’s only peers are the aforementioned quartet, which is pretty damn incredible.

As Shaq passed up the Dominiques, Hakeems and Moses’ of the world seemingly every week, Suns fans barely looked up from their popcorn. They’d give a nice golf clap, but it didn’t really mean as much as if say he had done this with the Lakers, a team he had won championships with and a fan base that saw him at his very best.

Even if not in a statistical sense of passing all-time greats, Suns fans (even those who support a trade) will feel that way if Steve Nash ends his career in another uniform.

We saw Nash as a skinny rookie, took a few years off from him when he became an All-Star in Dallas and then watched him become a transcendent player who won a pair of MVPs as a 6-foot-3 point guard right before our very eyes.

Suns fans will always love Nash for what he was during that great run from 2004-07, when his Suns changed the game of basketball, whereas another team’s fan base will only care for him in terms of what he brings to the table in the present.

Judging by tonight’s appearance as a correspondent on the Letterman show, we will certainly be seeing a lot of Nash on the airwaves after he retires regardless of where those final days are spent.

He was downright funny doing his reporter shtick and looked completely natural in that role. I’d say he can even skip that broadcasting boot camp Shaq attended a few weeks ago.

But he did end the show by saying something to the live New York studio audience that will make Phoenix fans queasier than watching the Big Unit win 300 with “San Francisco” across his chest when asked if he’d like to play for the Knicks.

Nash’s response typifies the modern-day athlete starting to near the end.

“I’d love to. Can you work it out?”