The Mechilta states that "there are ten songs" beginning with the song at the sea led by Moshe, and concluding with the tenth song which will be sung with Moshiach. All the [nine] songs mentioned in scripture are written in the feminine [shirah] since their rejoicing was followed by ["gave birth to"] further servitude. The tenth song of Moshiach is written in the masculine [shir] to indicate that it is permanent.
Chassidic teachings explain that the first nine songs emphasized primarily a desire to come closer to G-d from a distance, like a woman who longs to come closer to and receive from her husband. However, the tenth song of Moshiach will be sung from a feeling that G-d is already close and found openly in our midst, like a husband who is gracefully endearing himself to his wife.
Source: Sichas Shabbos Parshas Beshalach 5752, Lubavitcher Rebbe