The plain use is permission: 먹고 싶으면 먹어도 되고 "if you fancy it, go ahead", 먹고 싶으면 먹으라고 그래 "tell them to help themselves". Three lines show a different frame built on the same -면: 먹으면 먹을수록, "the more you eat, the more…", which pairs -면 with -ㄹ수록 to make a scale rather than a condition. 먹으면 먹을수록 마음에 안 들어 is "the more I eat it, the less I like it".