Lenina is a superficial "material girl." She is the essence of the New World Order: artificially happy, artificially young, sexually promiscuous, incredibly beautiful, and dumb as a box of rocks. Huxley even uses the word pneumatic to describe her. Pneumatic deals with air. It could mean that she is light and airy without a care in the world (true) or that she is full of air (airhead; also true). Lenina does not really know any emotions. So things like love or a maternal instinct are completely foreign to her. All that matters to her is her looks, others' looks, and material stuff.
That is why she is so attracted to John. She sees John the first time and thinks "…such a nice-looking boy, […] and a really beautiful body." John becomes her next conquest and when John lets her know that he is not interested, Lenina pursues that much more. She wants what she can't have.
Lenina doesn't like Linda for the very same reason that she likes John. Her only assessment of John is based on his looks. She applies this same reasoning and response to Linda. Linda has aged, her skin has wrinkled, and she has gained weight. Those things are completely foreign and abhorrent to Lenina.