Elizabeta and Gilbert were starting to grow tired of Roderich. The Austrian just wouldn't confess his feelings for the Hungarian. The Prussian wanted to see them be happy together... So he pretended to be Elizabeta's boyfriend to make Rod jealous and confess to Elizabeta... It worked.

Later, Henry was struggling to tell Gilbert how he felt. Elizabeta did what Gilbert once did for her... She pretended to be Gilbert's girlfriend make Henry jealous. Once again, it worked.