Melody can be crucial to the lyrics of a song for a number of reasons. Rising pitch literally provide a more "upbeat" feel, while notes heading downward tend to quiet and calm us. When the lyrics match the interval direction, we have a consistency that pleases our minds and our ears. If we think of lyrics as telling a story, which so many do, a good melody is like the rising and falling action in a story.
Much of this is mostly intuitive to us as human beings. Thinking about this, discussing the effects of pitch and rhythm is enlightening.