He doesn't know what makes him glance towards this woman and not another: maybe he recognizes the look in her eyes, of someone who had to grow up too fast, who had to protect others while young enough to need protection oneself, who wants to stop worrying for a while and have a little fun.
She doesn't know what makes her glance towards this man and not another: maybe she recognizes the look in his eyes, of someone who's lost more than anyone should bear, who has to stay to carry on the task they were set, who wants to lay down the burden for a few hours and be reminded of why it's worth carrying.
Sex is to create life, but it's also to make life worth living: these two might never meet again, might well forget the other's face and name, but for now, tonight, they're together, they're alive, and it's enough.