Why come back, Tony Gonzalez?
What do you have left to accomplish? You're already one of the (if not the) greatest tight ends of all time. You helped re-define the position. You have more than enough money than you will ever know what to do with. A little savvy saving and your family will be set for the rest of their lives as well.
Why come back to a league that has 4,000 of its former players suing because they felt they were misled on the dangers of concussions and that they were pressured (or even forced) to play through injuries that would risk both the quality and length of their lives? Football is a dangerous sport and it's also a young man's game. One hit could send anyone to the sidelines permanently. That same hit could make it awfully difficult to get out of bed in the morning for years afterward.
So, why come back?
In the end, Gonzalez is coming back for one final season (or maybe not final, we'll find that out next offseason) because he still wants to play football. This isn't a broke player who needs the veteran's minimum to survive. This isn't an aging has-been chasing down a record that some young kid is destined to break anyway. This isn't a sad legacy-kiling jaunt around the league's worst teams trying to latch on before the inevitable cut.
This is an elite player near the top of his game, long past his supposed prime who happened to be a major piece of the Atlanta Falcons' success in 2012. Oh, and by the way, this is a player who completely overhauled his training and nutrition regimen late in life finding his own personal fountain of youth.