You shouldn't tell your partner to lose weight.
Matt has noticed that his girlfriend Cindy has gained some weight.
Matt wants to tell Cindy what he thinks about her appearance.
Matt tells Cindy that she looks pretty today and delicately asks if she has possibly put on a few pounds.
Matt tells Cindy that she looks like she has gained a few pounds, she needs to go on a diet and lose some weight.
Cindy tells Matt that she didn't notice that she gained any weight, but maybe he's right.
Cindy gets upset and tells Matt that he made her feel really bad about the way she looks.