Concerning the tears, they deliberately select teary types, as noted
here.
"We look for people who need to get some weight off but we also look for someone who has a story to tell," she says. "If someone comes to us and says, 'I just love food, that's the only reason why I have put on this weight', then they're not for us. There needs to be an emotional journey for them to get through to their healing.
"We also look for people with a bit of character – introverts don't work very well on reality shows."
So if the person were quietly stoic about things, they wouldn't get on. They choose the weepy dramatic ones.
Concerning the health effects, most of the bad effects of quickly losing or putting on weight will be long-term rather than short-term. As well, most of the contestants are genetic freaks, in that they have to be basically healthy to get on the show. Most people who are that obese will have a whole swag of health problems, with sore joints, adult onset diabetes, heart disease and so on. Yet they don't have these issues. These people are as much freaks as the uncle everyone has who smoked a pack and drank a fifth of bourbon a day and lived to 96.
They thoroughly screen them for these issues, and most with such issues will be excluded from the show. So they'll be more physically resilient than the typical person who is that size.
They get tens of thousands of people apply to the show. They choose the ones who are mentally weak but physically tough. Thus tears but no heart attacks.