答案就是@Jason上面提到的。就是在App对象上设置user属性。

App.Xaml

public static User LoggedInUser { get; set; }

ViewModel.PersonalizeViewModel

public PersonalizeViewModel()
{
    _user = App.LoggedInUser;
    Username = _user.username;
    Email = _user.email;
    Password = _user.password;
    Shopping = _user.Shopping;
    Milk = _user.Milk;
    Loathes = _user.Loathes;
    Meat = _user.Meat;
    Fish = _user.Fish;
    Eggs = _user.Eggs;
    Vegetables = _user.Vegetables;
    Fruit = _user.Fruit;
}

查看.个性化.xaml.cs

public PersonalizeViewModel ViewModel { get; set; }

public Personalize()
{
    InitializeComponent();
    BindingContext = new PersonalizeViewModel();
}

查看.个性化.xaml

<Label Text="{Binding Username}" />

<Label Text="Email:" />
<Label Text="{Binding Email}" />

<Label Text="Meat:" />
<Label Text="{Binding Meat}" />

<Label Text="Fish:" />
<Label Text="{Binding Fish}" />

<Label Text="Shopping frequency:" />
<Label Text="{Binding Shopping}" />

<Label Text="Milk:" />
<Label Text="{Binding Milk}" />

<Label Text="Loathes:" />
<Label Text="{Binding Loathes}" />

<Label Text="Eggs:" />
<Label Text="{Binding Eggs}" />

<Label Text="Vegetables:" />
<Label Text="{Binding Vegetables}" />

<Label Text="Fruit:" />
<Label Text="{Binding Fruit}" />

<Label Text="Loathes:" />
<Label Text="{Binding Loathes}" />