Learn Croatian: Ponosimo se ljubavlju.

The reflexive verb ponositi se (to be proud of) requires its object to be in the instrumental case, with no preposition. So, even though the instrumental usually tells us what you are doing something "with", here it translates to being proud "of" something.

How does the word ljubav become ljubavlju?

Ljubav is a feminine noun that ends in a consonant. To make it instrumental singular, one of the endings we can add is -ju. When the final v of ljubav meets the j from the ending, they merge and insert an l to create the sound vlj. This sound change is called jotation.

Could I say ponosimo se ljubavi instead?

Yes! Feminine nouns ending in a consonant actually have two options for the instrumental singular: the jotated ending -ju (making ljubavlju) and the simpler ending -i (making ljubavi). Both are perfectly correct here. However, if you were to add an adjective, you usually have to use the -i ending, as in ponosimo se našom ljubavi (we are proud of our love).