The 2012 Olympic heptathlon champion, 30, announced last week she would not carry on to compete at next year's World Championships in London.
"My body can't cope with the volume of training that it used to," she told BBC sports editor Dan Roan.
"It was that fear of 'do I push on that one more year and potentially get injured and come away disappointed?'"
The 2009 and 2015 world champion, who had her first child, Reggie, in 2014, missed out on retaining her Olympic title in Rio by 35 points to Belgium's Nafissatou Thiam in August.
Ennis-Hill said immediately afterwards that she would not rush a decision over ending her career.
"Doing the heptathlon, you've got to be 100% motivated; you have to want to do the training or you're not going to get anywhere with it," she told BBC Sport.
"The past couple of years, I've struggled with injuries and Achilles problems. I'd have sessions that would go really well but then I'd be injured for a few days later.
"Do I say 'I'm in a really nice position here, I've achieved what I wanted to achieve and more' and do I walk away feeling really happy and satisfied?
"That's what I wanted to do."
"Doing the heptathlon, you've got to be 100% motivated; you have to want to do the training or you're not going to get anywhere with it," she told BBC Sport.
"The past couple of years, I've struggled with injuries and Achilles problems. I'd have sessions that would go really well but then I'd be injured for a few days later.
"Do I say 'I'm in a really nice position here, I've achieved what I wanted to achieve and more' and do I walk away feeling really happy and satisfied?
"That's what I wanted to do."
