The Day of Atonement focuses on repentance between man and God, atoning for the Sin of the Golden Calf which effected man's relation to God. The 15th of Av aims at worldly redemption, atoning for the Sin of the Spies who disavowed the land of Israel.
The enthusiasm which accompanies falling in love is actually meant to help a person take the monumental step of getting married. Such emotion, however, does not guarantee a happy marriage. Good traits and shared goals are much more important.