The NFL is a gladiator sport, and some guys just can't walk away. Father Time comes calling for all of us sooner or later, and the truly gifted athletes usually can manage to keep from falling head-first into old age faster than other people.
But at some point, every athlete has to face the fact he isn't as young as he used to be and that his body just isn't going to carry him through another season at the level it used to.
Aches and pains become bigger and more debilitating, injuries come easier and take longer to heal, and the toll of being a professional football player just finally catches up and says payment is due in full.
The great athletes usually heed the call and try to bow out gracefully once it becomes apparent they've lost a step.
Other guys just ignore that voice in the back of their head telling them it's over and try and fight through it, reasoning their natural talent will get them through another year or two.
Here are a dozen NFL players who just couldn't let it go.
Deion Sanders retired in 2001, but waffled, almost coming back in 2002 before deciding to stay retired, then coming back for a two-year stint with the Baltimore Ravens beginning in 2004. He was 37 at the time of his comeback.
His numbers were unremarkable during that time, but he wasn't bad.
As of this writing, Junior Seau still hasn't played since the 2009 season. It is unknown if Seau ever actually filed retirement papers with the league.
Seau was drafted in 1990 by the Chargers, had a great career, and then finished it off in Miami after the 2005 season. He then signed a one-day ceremonial contract to retire as a member of the San Diego Chargers.
After four days of pondering the meaning of life, Seau returned to football, playing three years with the Patriots. He did not play the 2010 season, but you never know when he might come back.
Franco Harris played for the Steelers from 1972-1983, and only was a few hundred yards short of breaking Jim Brown's all-time rushing yards record as the 1984 season dawned.
But Franco wanted a pay raise, and the Steelers, noting Harris definitely was showing his age, declined to give him a contract.
Eight games later as a member of the Seahawks, Harris retired, 192 yards short of the record.
After lighting things up in Buffalo throughout most of the 1970s, Buffalo traded the aging running back to the San Francisco 49ers prior to the 1978 season.
Instead of calling it a career, Simpson played two forgettable years before finally retiring and pursuing careers in acting, broadcasting and committing felonies.
He now resides at the Lovelock (Nev.) Correctional Center.
Ronnie Lott had a great career in San Francisco, and still was pretty good during his time with the Raiders, but age and injuries caught up with him.
He still was determined to play, though, and signed with the Jets in 1993, playing two average seasons there before trying to sign on with the Kansas City Chiefs in 1995.
He was injured during the preseason, tried to return to San Francisco, and then finally realized it was over and retired.
Yep, in this picture is an aging, hobbled, Joe Namath trying to give it one last go with the Rams.
The year was 1977, and Namath only lasted a few games as the starter before finishing the season on the sideline.
Johnny Unitas tore the muscles in his throwing arm during the 1968 preseason, and that was the beginning of a string of injuries that brought Unitas to the San Diego Chargers for the 1973 season.
He should have just retired.
Jerry Rice had 15 great years in San Francisco, but wanted more and got that in Oakland.
During the 2004 season, though, he was traded to the Seahawks, where he wore Steve Largent's number.
Other than that, there isn't a lot to be said for his time there.
Joe Montana spent his last few seasons in San Francisco injured, and then was traded to the Kansas City Chiefs for the 1993 season because Steve Young had claimed the starting job in San Francisco by that point.
Montana spent parts of 1993 injured as well, but played well when he was on the field. His 1994 season had a few really big highlights, including a duel with John Elway in which Montana prevailed, but the Chiefs were beaten in the Wildcard round by Dan Marino and the Dolphins, and Montana hung it up.
Emmitt Smith clearly had lost a step when Dallas let him go after the 2002 season.
But that didn't stop Smith from playing two more years for the Arizona Cardinals.
Herschel Walker clearly was on the downside of his career when he returned to the Cowboys in 1996, but that didn't stop him from trying to keep playing through the 1997 season.
While Walker was on the roster, he was barely used and finally retired after the 1997 season.
Now Walker said he'd like to make a comeback again at age 49 while promoting an upcoming MMA bout.
Stop. Just stop.
Brett Favre was forced out of Green Bay because he was aging and the Packers had a young quarterback named Aaron Rodgers ready to go.
Favre had one okay season with the New York Jets marred by an injury not revealed until after the season, and then a great year in Minnesota in 2009.
Then Favre REALLY should have retired, but just couldn't and came back.
While he practically had to be tied to the bench to keep him off the field, he was nowhere near as effective as he used to be and his diminishing skills were evident in poor decisions resulting in turnovers.
After a mostly injury-free career, Favre finally started missing time due to injuries as well. Favre currently says he is retired.