Lots of reasons;
Buying a house they really can't afford and having huge mortgage payments.
Expensive cars, clothing, furniture and electronics.
Also changing them when they are still in good condition, but they just get bored with them or think they have to get the latest design. So many people, for example, have to have the newest phone/tablet/laptop and change them at least yearly.
Going on expensive vacations.
Insisting on "updating" homes that are in good shape but just out of fashion or not perfectly suited to your taste. This attitude is pervasive thanks to all those renovation shows.
Legal bills from long, drawn out divorces.
The cost of child care.
Impulse buying.
Frequently eating out rather than cooking.
Alcohol. If most drinkers were to tally up the amount they actually spend on booze per month (in the pub, at home, in restaurants) they would probably be shocked.
Entertaining a lot. Again, if you tallied it up the numbers would probably suprise you.
Going to a lot of concerts, films, and other events.
Spoiled kids. Buying your kids practically everything they ask for.