Fancy
To take a sexual or romantic interest in someone; usually teenagers feel this emotion, and it roughly translates to 'I want to go out with her, but I won't really take this relationship seriously, or expect it to last that long'.

If one desires a serious, long-lasting relationship, they may be feeling love as opposed to fancying someone.
Teen 1: look at her, she's so fit! I really fancy her!
Teen 2: not keeping your expectations too high, eh?