It's ego, i think he has looked at what Bernard Hopkins has done in his 40's and thought to himself i can do that too.
Well he couldn't, he was washed up after Tarver 2, having been ruined by dropping 18lbs to go back to 175lb again. His chin was fine, but his legs were not. He might have gotten away with it if he was 25 years old not 35! No, it's ego IMO or just plain stupidity. Hopkins was a totally different fighter to Jones jr, he could tie you up, he was awkward to fight, he held a lot, he was frustrating to watch at times and i think even more frustrating to fight. No one really ever nailed him with a big shot too, so he was able to continue on much longer than expected due to his unique, awkward style. Jones jr was flashy, he relied on both leg and hand speed and evasiveness and when that goes you get nailed and that is exactly what happened. Unfortunately for Roy Jones jr it happened way to often.