Learn Spanish (Latin America): Me fascina que tú pintes tan bien.

Because me fascina expresses an emotion. Whenever you state a feeling about an action performed by someone else, you use que followed by the subjunctive mood. Since the subject changes from the person feeling the emotion to the person painting, the regular present tense pintas becomes the subjunctive pintes.

Why is it me fascina instead of yo fascino?

Fascinar works exactly like the verb gustar. The literal structure of the sentence is "That you paint so well fascinates me." The action of you painting is actually the subject doing the fascinating, and me is the object receiving that feeling.

Why use tan bien instead of tanto bien to say "so well"?

You use tan (meaning "so") before adjectives and adverbs like bien. You only use tanto (meaning "so much") before nouns or right after verbs. Since bien is an adverb describing how you paint, tan is the correct choice here.

Is the pronoun tú mandatory here?