Complex sentence: Ananda is often called the disciple of the Buddha who "heard much"; because he attended personally upon the Buddha and often traveled with him, Ananda overheard and memorized many of the discourses delivered by the Buddha to various audiences.
Split into simpler sentences: Ananda is often called the disciple of the Buddha who "heard much". Because he attended the Buddha personally and often traveled with him, Ananda overheard and memorized many of the discourses the Buddha delivered to various audiences.