Learn Swedish: Kärlek är viktigt.

Because this is a general statement about an abstract concept. Just like in English, Swedish uses the base form without an article when talking about abstract ideas like love, hate, or freedom in a general sense.

Kärlek is an en-word, so why does the adjective viktigt end in a -t?

This is a special Swedish rule for general statements! Even though kärlek is an en-word, the sentence describes the general concept of love rather than a specific physical item. When an adjective describes a general abstract idea or an activity, it always takes the neuter -t ending.

Is it ever correct to say Kärleken är viktig?

Yes, but it changes the meaning. If you say Kärleken är viktig (using the definite noun and dropping the -t on the adjective), you are talking about a specific, defined love, such as 'The love between them is important.' Our sentence, however, is about love as a universal concept.