The phrase "pouring his heart" is often used metaphorically to describe someone expressing their deepest emotions or thoughts, usually in a sincere, passionate, and unreserved way. It often implies being vulnerable or open about feelings, whether in a conversation, writing, or another form of expression. Example: - Literal sense: "He poured his heart into his letter to her," meaning he shared his most genuine emotions in the letter. - Figurative sense: "During the performance, he poured his heart into the song," meaning he sang with deep passion and emotion.

Metaphorically, love is often treated as liquid. The heart is a container for love. Just as you would pour water out of a pitcher, the heart can pour out love. Love "flows". The heart is "filled". Her love "spilled out". He was a "fountain of love". The opposite is also true. The absence of love can be treated as dryness: "He thirsted for love." "Their love withered to dust." "Her heart became like a desert."

To pour your heart out means to tell someone your secret feelings and things that worry you, usually because you feel a strong need to talk about them

'Pouring his heart out' (note the 'out') means he is expressing his emotions freely. Here it is presumably love that he is expressing, but it could be anything (well, usually tender or vulnerable emotions; we wouldn't use it about expressing anger).