Why is Lust a sin in Matthew, but praised in Proverbs?

Jesus says in the gospel of Matthew [paraphrasing] "if you look at a woman and lust for her, you have already sinned in your heart." In proverbs, they say, [again, paraphrasing] "let her [the companion of your youth] breasts satisfy you." Is that not lust? Aren't these two messages contradicting each other?