Why would having emotions imply that an entity is composite?

The *Summa Theologica* contains the following quote when considering "Whether God is composed of matter and form?": > Objection 2. Further, anger, joy and the like are passions of the composite. But these are attributed to God in Scripture: "The Lord was exceeding angry with His people" (Ps. 105:40). Therefore God is composed of matter and form. Aquinas rejects this view by saying that the team's description of God's emotions are anthropomorphic (it's referred to as anger because it has a similar effect to anger, for example). However, I do notice that he did not argue that something which has emotions is not necessarily composite, which suggests that he may not disagree with that statement. I'm somewhat baffled by it, though; why are emotions only characteristic of composite entities?