schwärmen für
to be crazy about
To feel great enthusiasm or a secret crush for someone or something.
Als Jugendliche hat sie für einen bekannten Schauspieler geschwärmt.
As a teenager, she was crazy about a famous actor.

to rave / swarm
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to be crazy about
To feel great enthusiasm or a secret crush for someone or something.
Als Jugendliche hat sie für einen bekannten Schauspieler geschwärmt.
As a teenager, she was crazy about a famous actor.

The verb 'schwärmen' is closely related to the noun 'der Schwarm' (the swarm). A 'Schwarm' is a large group of animals (e.g., bees, fish) or also people moving together.
Ein großer Schwarm Vögel flog über den Himmel.
A large flock of birds flew across the sky.
The preposition 'für' signals the meaning 'to be enthusiastic about'. When the subject is insects or birds, it refers to movement in a swarm. When people are the subject, it often means 'to stream somewhere in large numbers'.
Sie schwärmt für italienische Opern.
She is crazy about Italian operas.
If you want to express that someone is enthusiastic about a person or thing, always use the preposition 'für' followed by the accusative. A different preposition is wrong here.
Er schwärmt schon seit Wochen für seine neue Kollegin.
He has been crazy about his new colleague for weeks.
In the meaning 'to move in large numbers', 'schwärmen' is often combined with the separable verb 'ausschwärmen' (to swarm out) or with directional phrases like 'in die Stadt' (into the city) or 'in alle Richtungen' (in all directions).
Am ersten sonnigen Tag schwärmen die Menschen in die Parks.
On the first sunny day, people swarm into the parks.
to love
'lieben' and 'schwärmen für' can both express strong positive feelings. 'lieben' is stronger and deeper, especially with people in family or romantic relationships. But it can also express a very strong preference for things or activities. 'schwärmen für' often sounds lighter, more enthusiastic, or a little dreamy. With people, it can also mean a secret or not entirely serious crush. Learners may confuse the words because both show enthusiasm. For deep affection, use 'lieben'; for enthusiastic talking or a lighter crush, 'schwärmen für' fits better.

Seit der Reise schwärmt Moritz für die kleinen Städte am Meer.
Since the trip, Moritz has been crazy about the small towns by the sea.
Herr Becker liebt seine Familie und verbringt am Wochenende viel Zeit mit ihr.
Mr. Becker loves his family and spends a lot of time with them on the weekend.