Losing weight in a relationship?

He looks good but he's no longer like his former self. He's healthy and seems happy but his weight loss and lanky figure is hard to not notice. He was not big not thin. Just the perfect size. With heavy legs and you could see his pecs from his t shirt. His face was also chubbier. Ever since he's gotten into the relationship, he's lost weight, even from his thighs and legs, and face, and looks very thin. His face has also thinned out and gotten longer.

How can a happy relationship lead to this? He looks like a shell of his former self.
Losing weight in a relationship?
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