Learn Italian: Sei in ritardo per colpa della tua pigrizia, non a causa dell'ingorgo! - You are late because of your laziness, not due to the traffic jam!

Both translate to "because of", but they have a slightly different feel. per colpa di literally means "by fault of" and is used when assigning negative blame, like to someone's laziness. a causa di is a more neutral "due to", used here for the objective event of a traffic jam.

Why do we see della and dell' instead of just di after these compound prepositions?

The compound prepositions per colpa di and a causa di end with di. When the following noun takes a definite article, di must merge with it. Here, di + la (for la tua pigrizia) becomes della, and di + l' (for l'ingorgo) becomes dell'.

Why is it della tua pigrizia and not just di tua pigrizia?

In Italian, possessives almost always require a definite article, so "your laziness" translates to la tua pigrizia. Because of this, the preposition di has to combine with la to make della. The main exception where you drop the article is with singular family members, which doesn't apply to a word like pigrizia.

Is it always sei in ritardo for "you are late"?