Learn German: Er heiratet sie aus Liebe. - He is marrying her out of love.

Both prepositions express a cause or reason, but aus is used for deliberate or intentional actions motivated by an internal feeling. He is actively choosing to marry her because of his love. You use vor for involuntary physical or emotional reactions, like shaking from fear (vor Angst) or jumping for joy (vor Freude).

Why is there no article before Liebe?

When you use aus or vor to state a general emotion or abstract noun as the cause of an action, you drop the article (this is called the zero article). It is simply aus Liebe (out of love), rather than aus der Liebe.

Is Liebe still in the dative case even without the article?

Yes! The preposition aus always forces the noun into the dative case. Even though you can't see the article, the dative case is still active. If you were to add an adjective to describe the love, it would take the dative ending, such as aus großer Liebe (out of great love).

English speakers often want to say "He is marrying to her" or "with her". Why is it just heiratet sie?