In diatonic harmony and tonal music, certain scale degrees have an aural tendency to "want to go" to other scale degrees.

The strongest tendency tone is the 7th scale degree or "leading tone". It exhibits an unusually strong tendency to "go to" or resolve to the 1st scale degree, or "tonic". This is why, in pop music or church chorales and hymns, when you hear a V7 chord it makes sense and even relaxes your ear when it goes back to the chord the whole thing started in. 7 goes to 1, and your ear will always want to hear it go to 1, especially if it is sustained.

The other strong tendency in tonal music for 4 (subdominant) to resolve down to 3 (mediant). This happens often enough but not with the regularity that 7 moves to 1.

The major exception concerning the 7->1 movement is when the melody descends from 1 in a conjunct motion, 1->7->6->5, as in the first four notes of "Joy to the World".