I think I agree with tamuz, that it's about wishing a girl would like you, or notice you, for that matter. but I think the term 'sick and selfless' means that he's given so much of his energy (and himself, for that matter) into this girl. He's given too much. he hasn't gotten anything back from her, therefore he becomes mentally ill. the sick in sick and selfless.
I think I agree with tamuz, that it's about wishing a girl would like you, or notice you, for that matter. but I think the term 'sick and selfless' means that he's given so much of his energy (and himself, for that matter) into this girl. He's given too much. he hasn't gotten anything back from her, therefore he becomes mentally ill. the sick in sick and selfless.