It's rude to point out a partner's flaws.
Mary has been dating her boyfriend for a year.
Mary wants her boyfriend to stay with her.
Mary asks her boyfriend to start going to the gym with her, so they can both lose weight.
Mary tells her boyfriend he's fat and nobody else will want him.
Mary's boyfriend gets healthier and appreciates having someone to cheer him on.
Mary's boyfriend becomes depressed and sick from starving himself and breaks up with her.