"Because I was fat," he says, grinning. "I used to be massive when I was younger. The only reason I got into boxing was with the intention of losing weight. I was the same weight at 13 as I am now. In fact, I was probably heavier."
Gorbals boxer Ham will contest the 56kg bantamweight division at the 2014 Commonwealth Games after cementing his Team Scotland place with a strong performance at the Boxing Scotland Finals in March, definitively winning his bout against Brandon Singh in a knock-out.
"I've known for the past year that I'd done enough to qualify but couldn't express it," he says. "After the Scottish Finals, when I knocked the boy out, I put on Facebook: 'I'm going to the Commonwealth Games' but then I got into trouble for it. I was told I had to wait until after the official announcement. But I can put it on now. It feels amazing."
It marks a second consecutive Games for the 22-year-old who reached the final 16 in Delhi before exiting after defeat to Namibia's Sakaria Lukas.
"I was only a boy when I went there," says Ham. "I went senior too fast. My first senior tournament was the Commonwealth Games and it was a big ask. But you can't knock back a chance like that. I was up five points going into the last round. Then I can't remember the rest of the fight.
"I would say that losing in Delhi is the best thing to happen to me because I've learned so much from that. It taught me, don't be cocky. I got too carried away with it all and started doing things I shouldn't. It gave me a boot up the backside and now four years on I'm ready to go and take gold in Glasgow, my home city."
Ham oozes confidence and charm, a cheeky twinkle in his eye. He started out at Dennistoun McNair Boxing Club in the east end of Glasgow before moving to Hayfield ABC in the Gorbals to train under his father, Joe senior.
"We are some team," says Ham of their relationship. "We train so hard and reap the rewards. That's been five Scottish Championships I've entered as a senior and five golds. I've won nine Scottish titles in a row since I was 14. I'm a two-time British champion. I've done everything I can domestically.
"My dad did something like 15 amateur fights when he was younger. He was going to the gym so I went with him just to lose a bit of weight. I had no intention of going anywhere with it. I used to hate it. He would be punching me in the face. Now it's part of life. It's my job. You learn to love it."
Ham spent a year as part of the GB Boxing programme in Sheffield but departed in 2013. "Since I've left the British team I've come on so much which sounds weird I know, but it just doesn't suit some people," he says. "It's a great set-up down there but everyone is different. Going back and forward to training each week, all that travelling, I got a bit fed-up.
"I have a close family. If you are supported in the household then you are going to do well. My mum is so supportive as is my gran. I've got a cracking team around me."
Ham receives some funding through a two-year grant from sportscotland institute of sport as well as his sponsors Forge MOT and Dunfermine-based Simpson Builders. His father and mother Nicky add what financial support they can.
"My mum and dad have been good at letting me live off them but that can only last so long," he says. "Obviously I'm keen to get some other sponsors if I can because everything helps."
Ultimately, says Ham, the long-term goal to turn professional.
"I want to make money from this. I want a career," he says. "I was a joiner and quit an apprenticeship with six months to go to focus on going to the European and world championships. That was in 2012. You only get one chance. I could sit around on a building site when I'm 30 saying: 'I could have done this or that' or I could go out there and so it."
Ham hasn't ruled out turning professional after Glasgow 2014 but is reluctant to rush into anything. "I don't plan to make any big decisions straight away," he says. "After the Commonwealth Games the only thing I will be deciding is where I'm going on holiday in Ibiza. My mum and dad are getting married in September so I'll get all of that out of the way first.
"Then I'll have a break, sit down and think about it. I don't want to rush into making a decision. At the end of the day this is my career. I don't want to look back and wish I'd done things differently. I'll go to the Games, win my medal, take a month off to chill out, then I'll decide on what I want to do next."
His self-chosen nickname is "Dragon" and Ham fully intends to live up to that moniker.
"It's a good thing I can fight," he laughs. "If someone calls you Dragon, you have to back it up. You don't want to be 'drag-in' yourself out of the ring after you get beat. You have to win."
He is equally confident when asked about the longevity of his boxing career. "What Glasgow is going to do for me is build a platform for when I turn professional," he says. "I'm going to make a career out of this. I'm going to build on it and then I'm going to become a world champion."
Before then the bright lights of the SECC Precinct beckon with the Commonwealth Games boxing getting under way on July 25 and Ham is raring to go. "I'm ready," he says. "I will put everything into the preparation for Glasgow. I will leave no stone unturned. I will make sure I win a medal or leave everything in the ring trying.
"To get a Commonwealth gold medal - or any medal - you will reap the rewards. Me? I'm destined for big things."