John's love for Lenina is intense because it brings out so many deep emotions for him.  He is more emotionally involved with his feelings for her than she can even understand.  He is willing to make a life-long commitment to her by marrying her.  He is also willing to go off on some adventure like killing a lion for her; but, he says that he is also willing to sweep a floor for her.  No matter what the task, he is willing to show Lenina his love (228). 

On the other hand, he is also deeply tied to the darker side of love when he is disappointed with Lenina's actions and misunderstanding. Just like the saying that there is a fine line between love and hate, John crosses that line quickly in chapter 13 when the misunderstanding escalates and he is too inexperienced to control those emotions in a healthy, communicative way. In his efforts to control himself from "having" her, he becomes physically violent by grabbing her. The following example helps to clarify the subject:

    The Savage caught her by the wrists, tore her hands from his shoulders, thrust her roughly away at arms length.

     "Ow, you're hurting me, you're . . . oh!" She was suddenly silent.  Terror had made her forget the pain. Opening her eyes, she had seen h is face --no, not his face, a ferocious stranger's, pale, distorted, twitching with some insane, inexplicable fury" (231).

What other thing, but love, can bring out both extremes of emotion?

Analyzing characters also helps to understand the answer to this question. Check out the link below for more insight.