<Label><TextBlock>Lorem<LineBreak/>ipsum</TextBlock></Label>
You need to use TextBlock because TextBlock accepts as children a
collection of Inline objects. So you are giving the TextBlock element
three Inline items: Run Text="Lorem", LineBreak, and Run Text="ipsum".
You can't do:
<Label>Lorem<LineBreak/>ipsum</Label> because Label accepts one Content child element.
Edit: Also, not sure exactly what your use case is but notice I
placed a TextBlock inside your Label element. Is it repetitive? Not
really, depending on your need.
Here's a good article on the differences
between the two elements:Difference between Label and TextBlock http://joshsmithonwpf.wordpress.com/2007/07/04/differences-between-label-and-textblock/